The tabulated occurrences of stratigraphically important and relatively common diatom taxa are shown in Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11, Table 12, and Table 13, including the sample depth both of meters below seafloor depth (mbsf) and meters composite depth (mcd). Table 14 and Table 15 provide the sample, meters below seafloor depth (mbsf), and meters composite depth (mcd) constrained to stratigraphic events identified for Sites 1010 through 1022, located along the north-to-south transect, whereas Sites 1012, 1013, 1015, and 1017 were situated in the low-biosiliceous-productivity region along the California Borderland basins where a good record of calcareous microfossils was expected. The composite depth assignment follows that reported in Lyle, Koizumi, Richter, et al. (1997).
Site 1010 is the deep-water site (3465 m) of the southern depth transect (Baja Transect) and is situated on 14- to 15-Ma basaltic basement. Six holes were cored at Site 1010 in the southern region of the California Current. The sedimentary sequence recovered at Site 1010 consists of an apparently continuous 210-m-thick interval of Quaternary to middle middle Miocene sediments.
The section includes an upper 82-m-thick sequence containing trace to absent diatoms, from the Quaternary through the latest Miocene. Paleomagnetic studies obtained a detailed magnetostratigraphy from the Brunhes to the top of Chron C3Bn (7 Ma). In the lower part of the interval, Chron C3A can be recognized from 61 to 75 mbsf, indicating an age assignment of 5.89-6.94 Ma. This is underlain by a 48-m-thick sequence of late Miocene to late middle Miocene age marked by variable but often common diatoms and an almost complete absence of planktonic foraminifers. This, in turn, is underlain by a 55-m-thick sequence of rapidly deposited diatom ooze of middle middle Miocene age.
Diatoms are rare to abundant and moderately well to well preserved throughout the middle Miocene through upper Miocene section (Table 4). Samples 167-1010C-10H-3, 20-21 cm, through 11H-3, 20-21 cm, are placed in the Thalassionema schraderi Zone (NPD 6B), where the topmost sample indicates not only the LCO of T. schraderi but also the end of high production in diatom assemblages at the middle late Miocene. Although the deep-sea hiatus NH6 aged 7.4-8.4 Ma (Keller and Barron, 1987), is widespread in the North Pacific (Barron, 1980a), this event was not found because of the poor preservation of diatoms in both Holes 1010C and 1010E.
Both the LO of D. katayamae and the LCO of D. simonsenii are recorded simultaneously in Sample 167-1010C-11H-5, 21-22 cm, which marks the top of the D. katayamae Zone (NPD 6A). The boundary between the D. katayamae Zone and the underlying D. dimorpha Zone (NPD 6A/5D) is assigned between Samples 167-1010C-11H-CC and 12H-3, 20-21 cm, based upon the LO of D. dimorpha. Zones NPD 6A through 5D (8.6-9.9 Ma) are greatly compressed from 97.2 to 103.7 mbsf, especially the unique horizon of 103.7 mbsf, which represents the total range of D. dimorpha in Hole 1010C. The FO of D. katayamae, which should be the uppermost event settled within Zone NPD 5D, coincides with the base of overlying Zone NPD 6A.
The interval from Samples 167-1010C-12H-5, 21-22 cm, through 14H-5, 20-22 cm, is assigned to the Thalassiosira yabei Zone (NPD 5C). The LCO of Denticulopsis praedimorpha in Sample 167-1010C-14H-CC marks the boundary between the T. yabei Zone and the underlying D. praedimorpha Zone (NPD 5B). A special feature of Site 1010 was the recovery of an unusually thick middle Miocene section assignable to the interval of the D. praedimorpha Zone (NPD 5B) through the underlying Crucidenticula nicobarica Zone (NPD 5A). The FO of D. praedimorpha, defining the base of Zone NPD 5B, is assigned in Sample 167-1010C-17H-CC. The interval of Zone NPD 5B is equivalent to the total range of D. praedimorpha definitely, having an estimated age of 11.5-12.9 Ma. Correspondingly, the same two zones were also observed in the 40-m-thick interval from Samples 167-1010E-14H-CC through 18H-CC. Assigned ages based on diatoms and other microfossil groups are similar between Holes 1010C and 1010E.
The coincidence of the LCO of D. hyalina with the FCO of D. simonsenii is assigned between Samples 167-1010C-19X-CC and 20X-3, 75-76 cm, and marks the boundary between the C. nicobarica Zone (NPD 5A) and the D. hyalina Zone (NPD 4B). Based upon the FO of D. simonsenii, Zone NPD 4 can be divided into two subzones (Yanagisawa and Akiba, 1998), but the basal barren sequence from Sample 167-1010C-20X-CC through 23X-CC interrupts a full and particular investigation at Site 1010. Though D. simonsenii are not found at all, the lowest sample can be placed within Subzone NPD 4Bb because of the common occurrence of D. hyalina without the presence of D. lauta. Likewise in Hole 1010E, the lowest Sample 167-1010E-19X-CC contains D. hyalina and can be correlated with the D. hyalina Zone (NPD 4B). The base of the sedimentary sequence at Site 1010, probably corresponding to the middle middle Miocene D. hyalina Zone, exhibits an age between 13.1 and 14.6 Ma.
Barron (1985b) noticed that the FO of D. "hustedtii" (= simonsenii) is too diachronous to be useful in lower middle latitudes. The first consistent dominance of D. "hustedtii" over D. hyalina is widely known as the dominance transition of D. hyalina to D. "hustedtii" and appears to be isochronous in the middle- to high-latitude North Pacific. Because of the latitudinal diachronism of the FO of D. "hustedtii," Barron (1985b) speculated that the FCO of D. "hustedtii" at middle-to-high latitudes probably corresponds with the FO of D. "hustedtii" in the warmer low-latitude Pacific. In Hole 1010C at 29°57.90´N, the FO of D. simonsenii is coincident with the abrupt transition from D. hyalina to D. simonsenii, which agrees with stratigraphic occurrences of these taxa in Deep Sea Drilling Project (DSDP) Leg 63, Hole 470 at 28°54.46´N (Barron, 1981).
Diatom datum levels indicate continuous and relatively rapid sediment accumulation rates within the middle Miocene, when the subarctic North Pacific assemblages are dominated typically by Denticulopsis simonsenii, D. praedimorpha, and D. hyalina. Furthermore temperate to subtropical indexes such as Annellus californicus, Coscinodiscus lewsianus, Craspedodiscus coscinodiscus, and Crucidenticula nicobarica persistently occur from Zone NPD 4 through the lower part of Zone NPD 5B.
Thalassiothrix longissima and Thalassionema nitzschioides are especially abundant in the middle middle Miocene sequence and are indicative of high oceanic productivity associated with an intense upwelling province. Abundant occurrences of Coscinodiscus marginatus and Proboscia barboi, suggesting the coastal upwelling conditions, break out sporadically in harmony with the persistence of the oceanic mass production. The rapidly deposited diatom oozes of middle middle Miocene age are indicative of extensive upwelling of cool waters associated with the California Current.
Because of the modern distribution of Coscinodiscus marginatus and Proboscia barboi in the North Pacific (Sancetta and Silvestri, 1986; Takahashi, 1986) and the increased abundances of C. marginatus in the Pliocene (Barron, 1995), both robust species are indicative of coastal upwelling along the California margin. Takahashi (1986) studied seasonal fluxes of pelagic diatoms at station PAPA (50°N, 145°W, water depth 4200 m). He noted that C. marginatus is apparently adapted to a high nutrient supply and a quite large range of optimal temperature (5°-13°C) for high flux. The taxon maintains a consistent level of fluxes even outside the major peak in November and December, suggesting a relatively high level of tolerance to varying environmental conditions. Consequently, C. marginatus is considered to be an indicator of autumn/winter conditions as well as a probable cool-transitional to subarctic form.
Barron (1995) arranged the two kinds of diatom temperature indexes reconstructing warmer surface-water temperatures of the mid-Pliocene more than at the recent northwest Pacific. He presented that extended transition zone characterizing the middle-to-high latitude North Pacific, and C. marginatus was less frequent in mid-Pliocene surface waters to the north of the transition zone, in the same manner as in the modern North Pacific. Presumably, abundant C. marginatus and high sediment accumulation rates along the California margin are indicative of high productivity caused by coastal upwelling. C. marginatus, however, is robust and is indicative of concentration by dissolution. Burckle et al. (1992) noted the possibility that a number of C. marginatus are enhanced in selected samples by increased dissolution of more delicate taxa, and Barron (1995) also regarded that higher (>40%) relative abundances of C. marginatus may be the result of dissolution. Proboscia barboi has less data than C. marginatus, but their diatom valves are heavily silicified in fossil assemblages. Proboscia barboi, then, is expected to offer similar features in both thanatocoenosis and sea-surface community.
Age vs. depth plots provide data on changes in sedimentation rates in the sequence. A clear upward reduction in sedimentation rates is shown by all microfossil datums in the late middle Miocene at ~12 Ma, responding to the weakening of oceanic upwelling. The floras indicate a severe reduction in upwelling near the end of the middle Miocene, then late Miocene diatoms provide a great insight into the characteristics of assemblages associated with occasional coastal upwelling. Common and continuous occurrences of Coscinodiscus marginatus at Site 1010 might show manifest evidence of coastal upwelling in the warmer transition zone. During parts of the late Miocene through Pliocene, diatoms indicate a greater influence of temperate warm waters, because the scarcity of such cold-water genera as Neodenticula and Thalassiosira makes the interpretation of the North Pacific zonation quite impossible.
Site 1011 is the landward site of the Baja Transect, which crosses the California Current at ~30°N. The sedimentary sequence recovered from the five holes cored at Site 1011 in Animal Basin consists of an apparently continuous 281.5-m-thick interval of upper Miocene to Quaternary sediments, with no hiatuses. Dominant cold-water species of planktonic foraminifers, diatom, and radiolarian assemblages exhibit evidence of strong upwelling conditions during the late Mio-cene. The predominance of temperate foraminiferal species in early late Pliocene through early Pleistocene assemblages, and rare occurrences of diatoms and radiolarians, indicate warm-temperate to cool-subtropical conditions with a weakening of the upwelling system. Diagenetic sulfate reduction in these highly organic-rich sediments caused the dissolution of fine-grained magnetic minerals, which prevented the establishment of a magnetostratigraphy.
Above 146.5 mbsf in all holes, throughout the late early Pliocene to Quaternary, diatoms are rare to barren, and large quantities of clay, clastic material, and calcareous nannofossils continuously dominate the lithostratigraphic succession. This is underlain by a 64-m-thick sequence of late late Miocene to early Pliocene age, which is characterized by rare and sporadic diatoms and radiolarians as well as generally uncommon assemblages of planktonic foraminifers and few to abundant calcareous nannofossils. Below this interval, a 61-m-thick sequence of rapidly deposited diatom-rich sediments of late Miocene age was recovered.
Diatoms are abundant to common and display excellent to good preservation in chiefly the lowermost seven cores of Hole 1011B (210.5-271.7 mbsf). Diatom assemblages are typical of the subarctic region of the North Pacific and can be zoned readily using the North Pacific diatom zonation (Table 5). The interval from Sample 167-1011B-19X-CC through 23X-3, 20-21 cm, corresponds to the Rouxia californica Zone (NPD 7A) based upon sporadic occurrences of R. californica. Persistent reworking of early late Miocene forms of D. katayamae, D. dimorpha, and conceivably D. simonsenii occur within the uppermost Miocene NPD 7A Zone. The LCO of Thalassionema schraderi is recognized in Sample 167-1011B-23X-5, 20-21 cm, and marks the top of the T. schraderi Zone (NPD 6B).
A complete sequence from the T. schraderi Zone (NPD 6B) down to the Denticulopsis dimorpha Zone (NPD 5D) was recovered through Samples 167-1011B-23X-5, 20-21 cm, to 29X-CC, which falls within the middle late Miocene. The coincidence of the LCO of the Denticulopsis simonsenii group and the LO of Denticulopsis katayamae in Sample 167-1011B-26X-CC marks the boundary between the Thalassionema schraderi Zone (NPD 6B) and the underlying Denticulopsis katayamae Zone (NPD 6A). Akiba (1986) adopted the former event dated at 8.6 Ma as the zonal boundary because of the practical difficulty in morphological identification, whereas Yanagisawa and Akiba (1998) used the latter at 8.5 Ma as was defined originally by Maruyama (1984b). If a careful examination is allowed, the latter should be perfect as the boundary. But from the point of view of realistic decision making, support for the former should not be abandoned immediately. Above these events, the FCO of T. schraderi (8.4 Ma) is assigned to Sample 167-1011B-25X-CC.
The top of the Denticulopsis dimorpha Zone (NPD 5D) has been placed between Samples 167-1011B-28X-5, 21-22 cm, and 28X-CC at the LO of D. dimorpha at 9.16 Ma. The presence of D. dimorpha accompanied by D. katayamae in the lowest sample, 167-1011B-29X-CC, indicates that the base of the cored section at Site 1011 is assuredly younger than 9.26 Ma, an assignment that is consistent with that of the radiolarians.
Assemblages examined from Samples 167-1011B-23X-5, 20-21 cm, through 28X-3, 21-22 cm, are commonly dominated by Coscinodiscus marginatus, a large, robust centric diatom that is resistant to dissolution and transportation. Long needle-like diatoms such as Thalassionema nitzschioides, T. schraderi, and Thalassiothrix longissima occasionally exhibit abundant occurrences in accord with C. marginatus. These samples fall just below the 64-m-thick, diatom-scarce interval and represent an almost entire sequence of the NPD 6B through 6A zones. Increased abundances of C. marginatus shows evidence for strong upwelling conditions during the late Miocene at Site 1011.
On the other hand, the diatom-poor sequence from the latest Miocene to the early Pliocene indicates warm-temperate to cool-subtropical conditions with a weakening of the upwelling system. Although cold-water Neodenticula species always dominate diatom assemblages wherever subarctic zonation prospered in the late Miocene through Pleistocene age, they are completely absent, nor are there characteristics of temperate to subtropical markers in Hole 1011B. Moreover, the scarcity of diatoms in the clay-rich, upper part of the sequence at Site 1011 as well as Site 1010 results from ecological exclusion or chemical decomposition from the relatively warm coastal waters during the late Pliocene through Quaternary.
Sites 1012, 1013, 1015, and 1017 comprise the Pliocene through Quaternary sequences with very little diatoms, so they are explained together. Stratigraphic documentation of diatom fossils are presented in the Leg 167 Initial Reports volume (Lyle, Koizumi, Richter, et al., 1997).
The sedimentary sequence recovered from the two holes at Site 1012 consists of an apparently continuous 264-m-thick interval of upper lower Pliocene to Quaternary sediments. Also, three holes were cored at Site 1013, recovering an apparently continuous sequence of late late Pliocene (2.7 Ma) through Quaternary age. Site 1012 is located at a distance 105 km from the shore, and similarly Site 1013 is 115 km off the coast. Diatoms are barren to few in all sections from the Pliocene through Quaternary interval at both Sites 1012 and 1013. All diatom assemblages are poorly preserved, and neither the North Pacific diatom zonation nor the standard diatom datum levels were recognized in any part of the holes.
The poorly preserved assemblages are commonly accompanied by abundant sponge spicules. Large, robust, and fresh biosiliceous skeletons are resistant to dissolution and transportation. Diatoms recovered from both sites contain consistent, but typically few, reworked planktonic diatoms from pelagic middle Miocene sediment sequences. These specimens include many diagnostic cool-water taxa such as Actinocyclus ingens and Denticulopsis spp. Persistent and scattered common occurrences of reworked specimens as well as numerous sponge spicules indicate strong incursions of marginal shallow waters that are associated with seafloor erosion from Baja California during the latest Cenozoic or from the tops of California Borderland ridges.
Site 1015 in the Santa Monica Basin is the only Leg 167 site located within an inner borderland basin at a water depth of 901 mbsl. Diatoms are either absent or occur only in trace amounts throughout the upper Quaternary sequence at Site 1015. Reworked Miocene taxa are very poorly preserved and are of no value for biostratigraphy through the distal turbidites extended onto Site 1015. The uppermost core-catcher samples of Holes 1015A and 1015B (Samples 167-1015A-1H-CC and 167-1015B-1H-CC) contain common to few siliceous bands dislodged from the girdles of diatom cells.
Site 1017 is located about 50 km west of Point Arguello on the continental slope at 955-m water depth. It is the shallow-water drill site in the Conception Transect (35°N), near an important upwelling center off Point Conception. Five holes were cored to a maximum depth of 204.2 mbsf, recovering an apparently continuous interval of Quaternary age.
Site 1017 exhibits a characteristic biosiliceous component made up of trace amounts of fragmented diatoms, rare reworked diatoms of middle Miocene age, and common fresh sponge spicules. Thus, Site 1017 resembles Sites 1012, 1013, and 1014. These mixtures suggest a persistence of reworking of relatively shallow-water components during the Pleistocene. Although sparse occurrences are documented of Actinocyclus oculatus in Sample 167-1017B-11H-CC and of Probosia curvirostris in Sample 167-1017B-12H-CC, diatom biostratigraphy is ineffective at Site 1017.
Site 1014 is located in Tanner Basin, within the outer band of California Borderland basins. Four holes were cored with the advanced piston corer (APC) and extended core barrel (XCB) to a maximum depth of 449.0 mbsf. The sedimentary sequence recovered from Site 1014 consists of a well-dated, apparently continuous, 325-m-thick interval of upper Pliocene to Quaternary sediments, underlain by a relatively poorly dated 124-m-thick sequence of lower Pliocene to possibly latest Miocene age. The sediments consist predominantly of calcareous nannofossils, foraminifers, and siliciclastic clays. A well-constrained biostratigraphy and chronology for all holes at Site 1014 is provided by a combination of calcareous nannofossil, planktonic foraminifer, and radiolarian datums for the upper Pliocene and Quaternary.
Few diatoms occur in the Pliocene and Quaternary section at Site 1014 (Table 6). All diatom assemblages are poorly preserved, and, as at Sites 1012 and 1013, neither the Leg 167 North Pacific diatom zonation nor the standard diatom datum levels was recognized in Holes 1014A and 1014B. Poorly preserved assemblages are commonly accompanied by abundant fresh sponge spicules, rare radiolarians, and reworked planktonic diatoms, forming an unusual biosiliceous assemblage.
A stratigraphic interval younger than Sample 167-1014A-12X-CC contains assemblages lacking Actinocyclus oculatus, and is assignable to the Proboscia curvirostris Zone (NPD 11) through the N. seminae Zone (NPD 12). Sparse occurrences of A. oculatus throughout Samples 167-1014A-13H-CC to 38H-CC suggest weakly a range of late Pliocene to early Pleistocene (~4.0-1.0 Ma), and a possible boundary between Zones NPD 10/11 is indicated by the last occurrence (LO) of A. oculatus at 1.01-1.46 Ma in Sample 167-1014A-13X-CC (112.4 mbsf). Paleomagnetic investigation revealed a good magnetostratigraphic record down to 100 mbsf and allowed the identification of the Brunhes and Jaramillo normal-polarity intervals. The chronostratigraphic boundary of Chron C1r.1n (Jaramillo onset) at 1.07 Ma is reported from 86 to 87 mbsf at both Holes 1014B and 1014D, suggesting that the evaluation of the age of the LO of A. oculatus is not completely incorrect but narrowly reasonable.
The diatom assemblage in Sample 167-1014A-39X-CC contains Hemidiscus cuneiformis, Thalassiosira cf. leptopus, and Coscinodiscus sp., but lacks Neodenticula spp., Nitzschia reinholdii, and another Nitzschia sp. The assemblage is typical of neither the subarctic North Pacific nor equatorial East Pacific Ocean, but rather of the relatively warm-temperate surface water that might be extended along the west coast of California at the time. Moreover, subarctic index species such as Neodenticula kamtschatica, N. koizumii, and N. seminae occur in traces throughout the section, which indicate relative warmth around Site 1014 from the early Pliocene through the late Pleistocene.
More than 20 m of the section, from 179 to 199 mbsf, includes abundant to few specimens of both Coscinodiscus marginatus and Proboscia barboi (Samples 167-1014A-20X-CC through 22X-CC and 167-1014B-22X through 24X-CC). These diatom elements are so large and robust that their assemblage is resistant to dissolution and fragmentation. They are also characteristic of bordering upwelling (see "Site 1010" section, this chapter), indicative of high-productivity episodes during the late Pliocene on the continental margin (Table 6).
Persistent reworking of many diagnostic cool-water taxa such as Actinocyclus ingens, A. ingens v. nodus, and Denticulopsis spp., documented in the Pleistocene through upper Pliocene section from Site 1014, represents strong incursions of marginal shallow waters, inducing seafloor erosion. Reworked specimens, which are commonly dissolved and fragmented, suggest that they might be derived from the corresponding sediment sequences of Luisian through Mohnian age in the Monterey Formation of California (Poore et al., 1981).
During DSDP Leg 63 off southern California, Barron (1981) also reported a diatom-poor interval, containing only rare, poorly preserved specimens reworked from the Miocene, between lowermost Pliocene and upper Pliocene diatomaceous sediments at both Holes 467 (33°50.97´N) and 469 (32°37.00´N) near Hole 1014 (32°49.99´N). Barron (1981) also explained that a slackening of the California Current during a period of relative warming caused reduced upwelling with resultant decreased diatom productivity.
Site 1016 is located about 150 km west of Point Conception and forms the deep-water drill site (water depth of 3834 mbsl) on the Conception Transect (35°N). Site 1016 occupies an important transitional zone for modern diatom flora and provides material to investigate paleoceanographic conditions near the center of the California Current. The sedimentary sequence recovered from the four holes at Site 1016 consists of a well-dated, apparently continuous, 308-m-thick interval ranging from the upper Miocene to the Quaternary. At the base of Hole 1016A, cored to a maximum depth of 316.5 mbsf, slow drilling and poor recovery were encountered in massive chert and porcellanite layers starting at 298 mbsf. Magnetic intensities were below the noise level of the magnetometer and precluded the establishment of a magnetic polarity stratigraphy aboard ship.
Diatoms are generally common to abundant and poorly to well preserved in the upper Miocene through Pleistocene section above 290 mbsf at Site 1016 (Table 7). In contrast to Sites 1011 through 1015, typical Quaternary assemblages are well preserved and easily zoned from Cores 167-1016A-1H through 11H. The LO of the Proboscia curvirostris (0.30 Ma) falls between Samples 167-1016A-3H-CC and 4H-CC, where it differentiates the N. seminae Zone (NPD 12) from the underlying P. curvirostris Zone (NPD 11). The zonal boundary between the NPD 12/11 zones is also determined in Holes 1016B and 1016D. A rare occurrence of Rhizosolenia matuyamai in Sample 167-1016A-5H-CC is dated as 0.91-1.14 Ma and indicates a tight correlation with a short interval spanning the normal-polarity Jaramillo subchron (Barron, 1980b; Barron and Gladenkov, 1995). The solitary occurrence of R. matuyamai has been used to tentatively place the boundary between the P. curvirostris and A. oculatus Zones (NPD 11/10). The FO of P. curvirostris in Sample 167-1016A-6H-CC can be detected from the middle part of Zone NPD 10 accordingly. Because the Pliocene/Pleistocene boundary is placed between 55.1 and 55.8 mbsf by a combination of calcareous nannofossils and planktonic foraminifer biostratigraphy, the NPD 11/10 boundary in Hole 1016A should lie between Samples 167-1016A-4H-CC and 5H-CC. Consequently, the last rare occurrence of A. oculatus in Sample 167-1016A-7H-CC is stratigraphically too low to mark the top of the A. oculatus Zone (NPD 10).
Following Koizumi (1992), the base of the A. oculatus Zone (NPD 10) and top of the underlying Neodenticula koizumii Zone (NPD 9) are defined by the LO of N. koizumii (2.0 Ma). The LO of Thalassiosira convexa (2.4 Ma) occurs just below the LO of N. koizumii (Barron and Gladenkov, 1995). These latest Pliocene events were documented in Samples 167-1016A-11X-CC through 12X-CC, but occurrences of these species are too sparse to provide high stratigraphic reliability at Site 1016. Despite an abundance of diatoms in sediments between Samples 167-1016A-11X-CC (96.3 mbsf) and 31X-CC (288.8 mbsf), three zones of the late Pliocene through latest Miocene age—the N. koizumii Zone (NPD 9), the underlying N. koizumii -N. kamtschatica Zone (NPD 8), and the lowest N. kamtschatica Zone (NPD 7)—could not be clearly defined because of the absence of zonal marker species N. koizumii and N. kamtschatica. Barron and Baldauf (1986) reported that it is inappropriate to use the N. kamtschatica Zone (NPD 7) in California and substituted two zones defined by Barron (1981): the Nitzschia reinholdii Zone and the overlying Thalassiosira oestrupii Zone. The combination of calcareous nannofossils and planktonic foraminifers provided a well-constrained biostratigraphy and chronology for Hole 1016A, and fairly placed the lower/upper Pliocene boundary between 154.5 and 157.45 mbsf and the upper Miocene/lower Pliocene boundary between 182.8 and 192.3 mbsf.
The 190-m-thick diatomaceous interval from 96.3 to 288.8 mbsf at Site 1016 includes abundant representatives of Coscinodiscus marginatus, Proboscia barboi, Thalassionema nitzschioides, and Thalassiothrix longissima, which are characteristic of oceanic-to-marginal upwelling systems and indicative of high coastal productivity. The diatom assemblages contain scattered and consistent representatives of warm-water taxa such as Hemidiscus cuneiformis, Thalassiosira convexa, Azpeitia nodulifera, and N. reinholdii. These assemblages are typical of neither the subarctic North Pacific nor the equatorial eastern Pacific, but rather the relatively warm-temperate surface waters that extend along the northeastern Pacific rim off California.
Such diatom assemblages suggest two major episodes of strong upwelling during the upper Miocene (from 182 to 288 mbsf) and the upper Pliocene (from 96 to 154 mbsf). These two episodes are separated by an interval marked by low sedimentation rates during the early Pliocene (from 163 to 173 mbsf). Although age vs. depth plots for Holes 1016A and 1016B show high sedimentation rates from the Quaternary to the upper Pliocene and for the upper Miocene (Lyle, Koizumi, Richter, et al., 1997), sedimentation rates were drastically lower for the lower to mid-Pliocene, indicating decreased vertical advection of deep waters.
The coexistence of both N. reinholdii and N. pliocena in Sample 167-1016A-31X-CC at 288.8 mbsf indicates that the base of the diatomaceous interval in Hole 1016A may be younger than 7.3 Ma. Surface waters at Site 1016 have maintained intermediate properties between subarctic cold waters and subtropical warm waters during the latest Miocene through Pliocene. Oceanic-to-marginal upwelling caused high diatom productivity between both fronts along the eastern rim of the temperate North Pacific. An almost continuous absence of cold-water Neodenticula spp. reflects a transitional provincialism of the diatom flora, suggesting warm conditions during the late Miocene through late Pliocene.
Site 1018 is located about 75 km west of Santa Cruz, California, on a sediment drift just south of Guide Seamount at a water depth of 2477 mbsl. It provides continuity in the Leg 167 sites from the central California margin between the Gorda Transect at 40°N and the southern California transects. The sedimentary sequence recovered from the four holes at Site 1018 consists of a well-dated, apparently continuous 426-m-thick interval of uppermost lower Pliocene to Quaternary sediments. All the microfossil groups at Site 1018 are clearly dominated by cool, high-latitude elements throughout the late Neogene. Site 1018 is sufficiently far north in the California Current to exclude most to all subtropical elements, even during interglacial episodes. Diatoms are dominated by subarctic forms, however, with the addition of much less abundant temperate elements.
Diatoms are generally common to abundant and poorly to moderately well preserved throughout the Quaternary through upper Pliocene section recovered at Site 1018 (Table 8). In the uppermost parts of the Quaternary section, however, diatoms are few because of increases in clay minerals and silt grains. Diatom assemblages from all core-catcher samples consist almost entirely of oceanic species, occasionally including littoral benthic forms. They are mainly of the subarctic North Pacific Ocean, although such warm-water taxa as Hemidiscus cuneiformis, Nitzschia reinholdii, Fragilariopsis doliola, Azpeitia nodulifera, and Thalassiosira convexa are typically present throughout the section. The core-catcher samples examined range from the Quaternary Neodenticula seminae Zone (NPD 12) to the late Pliocene Neodenticula koizumii-Neodenticula kamtschatica Zone (NPD 8). The Neogene North Pacific Ocean diatom zones are readily distinguishable, and standard diatom datum levels were used to recognize these zones.
The boundary between the late Quaternary N. seminae and Proboscia curvirostris Zones (NPD 12/11) is clearly indicated by the LO of P. curvirostris around 70 mbsf in Holes 1018A, 1018C, and 1018D. This event is detected from between Cores 167-1018A-7H-CC and 8H-CC, 167-1018C-8H-CC and 9H-CC, and 167-1018D-7H-CC and 8H-CC. The continuous occurrence of F. doliola is observed from Zone NPD 11 down to the upper part of Zone NPD 10 (Samples 167-1018A-8H-CC through 15X-CC). This species is one of the most diagnostic forms within the low-latitude diatom zonation (Baldauf and Iwai, 1995), and its stratigraphic distribution probably indicates the proximity of warm surface waters.
The LO of Actinocyclus oculatus in Sample 167-1018A-11X-CC marks the boundary between the P. curvirostris and A. oculatus Zones (NPD 11/10) in Hole 1018A, but it is not detected in Holes 1018C and 1018D. Within Zone NPD 10, the FO of P. curvirostris between Samples 167-1018A-20X-CC and 21X-CC may be useful for subdividing this zone. This datum, with an assigned age of 1.58 Ma, is tentatively adapted as a zonal boundary between Zones NPD 11 and 10 (between Samples 167-1018C-20X-CC and 21X-CC) in Hole 1018C, because the LO of A. oculatus was not observed. Both paleomagnetic records and the short range of Rhizosolenia matuyamai in Sample 167-1018C-16X-CC, which ranges from only 0.91 to 1.14 Ma, supports this biostratigraphic determination. Positive paleomagnetic inclinations in the top 88 mbsf, which most likely represent the Brunhes Chron, approve of diatom sequence through Zones NPD 12 to 11. Below the normal polarity interval, an interpretation of the inclination record was not possible because of the low magnetic intensity and core disturbance by XCB coring.
Koizumi (1992) proposed that the base of the A. oculatus Zone (NPD 10) and top of the underlying N. koizumii Zone (NPD 9) be defined by the LO of N. koizumii. This latest Pliocene event at 2.0 Ma occurs between Cores 25X-CC and 26X-CC in both Holes 1018A and 1018C. The LO of T. convexa (2.35 Ma) is documented in the middle of Zone NPD 9 between Samples 167-1018A-31X-CC and 32X-CC. The LO of N. kamtschatica and its last common occurrence (LCO) are narrowly assignable between Samples 167-1018A-38X-CC and 39X-CC, and between 44X-CC and 45X-CC, respectively. Although Barron and Gladenkov (1995) adopted the LCO event as the top of Zone NPD 8, the LO of N. kamtschatica is so generally used by Akiba (1986), Koizumi (1992), and Yanagisawa and Akiba (1998) that here it was chosen to mark the top of the underlying N. koizumii-N. kamtschatica Zone (NPD 8) in Hole 1018A.
The lowest record of N. seminae in Sample 167-1018A-42X-CC is situated between the LO of N. kamtschatica and its LCO within the topmost of Zone NPD 8 in Hole 1018A. According to Yanagisawa and Akiba (1998), the FO of N. seminae should be defined by the FO of its closed copula and can be estimated at about 2.4 Ma above the LO of Neodenticula kamtschatica at DSDP Hole 438A off northeastern Honshu, Japan. However, Koizumi (1992) and Barron and Gladenkov (1995) reported that the FO of N. seminae coincides with the LO of N. kamtschatica in the Japan Sea and in the high-latitude western North Pacific, respectively. On the other hand, Koizumi and Tanimura (1985) recognized that event within the N. koizumii-N. kamtschatica Zone (NPD 8).
These discrepancies indicate both that the LCO and/or LO of N. kamtschatica are diachronous and appear to be earlier off California than in the subarctic region, and that N. seminae cannot be distinguished from N. koizumii based solely on valve morphology. Yanagisawa and Akiba (1998) note that the type of copula is a key to the final decision to differentiate clearly N. seminae, with a closed copula, from N. koizumii, with an open copula. Moreover, they report the presence of an evolutionary intermediate form referred to as Neodenticula sp. A by Akiba and Yanagisawa (1986), which makes it more difficult to differentiate the two species. The taxonomic refinement among three taxa of Neodenticula would contribute to biostratigrahic establishment across the Zone NPD 9/8 boundary, though the stratigraphic distribution of the closed copula of Neodenticula (= FO of N. seminae) was not investigated during Leg 167.
In contrast to the late Pliocene to Quaternary diatom assemblages at Site 1016, those of Site 1018 contain consistent, but typically few, neritic diatoms. These taxa include representatives of the littoral benthic genera Cocconeis, Cymbella, Epithemia, Eunotia, and Gomphonema and are taken as evidence for sediment transported from the west coast continental shelf or from the top of Guide Seamount, if the latter was within the photic zone. The transport of these diatoms must have been contemporaneous with higher input of terrigenous sediments because reworked diatoms are extremely rare in Site 1018 sediments.
In the uppermost Pliocene to lower Pleistocene, the diatom assemblages are marked by rare-to-common N. seminae, few-to-common P. barboi, and few-to-abundant Stephanopyxis species, which are indicative of high oceanic-to-marginal productivity. The abundant fragments of Thalassiothrix species, conspicuously common occurrence of C. marginatus, and large numbers of siliceous bands dislodged from diatom girdles sometimes found in this interval, especially during the latest Pliocene, may indicate surface water, reflecting coastal upwelling associated with climatic change.
Site 1019 is located about 60 km west of Crescent City, California, in the Eel River Basin at a water depth of 977 mbsl. It is the nearshore drill site of the Gorda Transect. The five holes at Site 1019 recovered an apparently continuous sediment sequence of the late Quaternary. The age of the base of Hole 1019C at 247.8 mbsf is well constrained at ~900 ka.
Two characteristic North Pacific diatom zones of the Neodenticula seminae Zone (NPD 12) to the Proboscia curvirostris Zone (NPD 11) were recognized in the Quaternary section at Site 1019. Diatoms are rare to abundant and poorly to moderately well preserved throughout the sequence (Table 9). The last consistent occurrence of P. curvirostris in Sample 167-1019C-5H-CC marks the boundary between the N. seminae and P. curvirostris Zones (NPD 12/11) in Hole 1019C. This boundary in Hole 1019D occurs slightly higher (between Samples 167-1019D-3H-CC and 4H-CC) than in Hole 1019C. Positive paleomagnetic inclinations of the top 75 mbsf most likely represent the Brunhes Chron and support these diatom zonation.
Radiolarians are completely dominated by subarctic forms, whereas diatoms are dominated by cool, high-latitude, North Pacific assemblages, with limited subtropical forms. The cold-water representative N. seminae occurs throughout most of the sequence and is indicative of Zone NPD 11 but not of Zone NPD 12. Despite the relatively high-latitude location of this site, warm-water taxa such as Fragilariopsis doliola and Nitzschia reinholdii are sporadically but persistently present in the section except in the uppermost Zone NPD 12. Fine-grained sand turbidites, which were observed to be more common in the latest part of the sequence younger than ~400 ka (above 42 mbsf), explain why diatom assemblages within Zone NPD 12 seem broken and reduced.
Both the persistent occurrence of P. curvirostris and the absence of Rhizosolenia matuyamai in the lowest part of the sequence (Samples 167-1019C-23X-CC to 25X-CC) indicate that an age of the base in Hole 1019C is younger than the condensed range of the latter species from 0.91 down to 1.14 Ma. Such distribution of diatoms cooperate with the evolutionary transition in a radiolarian bioseries between 199.7 and 190.1 mbsf, which is well dated between 0.9 and 0.8 Ma based upon magnetostratigraphic intercalibration.
Both N. seminae and Stephanopyxis species are characteristic of coastal upwelling and exhibit highly variable abundance throughout the sequence. Coincidence of common-to-abundant occurrences of these taxa, corresponding to high-productivity, seems likely to reflect paleoclimatic oscillations as glaciation expanded in the Northern Hemisphere.
Site 1020 is located on the east flank of the Gorda Ridge at a water depth of 3038 mbsl. It is about 170 km west of Eureka, California, and forms the deep-water drill site on the Gorda Transect. The site is located on an abyssal hill that trends northeast and rises ~50 m above the surrounding seafloor on 5.1-Ma ocean crust. The sedimentary sequence recovered from the four holes at Site 1020 consists of an apparently continuous 279-m-thick interval of upper lower Pliocene to Quaternary sediments. A well-constrained biostratigraphy and chronology are provided by a combination of paleomagnetic reversals and microfossil datums. Calcareous nannofossils indicate that the base of Hole 1020B is 3.79 Ma in age.
Except for a barren interval immediately above basaltic basement, all samples from Site 1020 were readily placed within the subarctic diatom zonation of the North Pacific (Table 10, Table 11). Sparse occurrences of temperate taxa, such as Hemidiscus cuneiformis, Nitzschia reinholdii, N. marina, and Thalassiosira convexa, indicate the influence of temperate water masses from the south during the late Pliocene through early Quaternary. Despite the location of Site 1020 in relatively deep water (>3000 m depth), diatom assemblages in the sequence above 140 mbsf are not typical of hemipelagic environments but show considerable resemblance to the assemblages encountered at nearshore Site 1019. The drill sites were situated almost certainly near the core of the northern region of the California Current during the critical time interval when the Northern Hemisphere glaciation developed. However, cold-water representatives belonging to the genus Neodenticula especially reveal low abundances throughout the sequence.
Diatoms are few to common and poorly to moderately well preserved throughout the upper Quaternary section recovered at Site 1020. Abundant clay diluted the diatoms, and almost all are strongly etched and fragmented by dissolution, except for robust forms of Stephanophyxis turris and S. dimorpha. In contrast, the lower Quaternary through upper Pliocene interval below 140 mbsf is diatomaceous, and diatoms in this interval are common to abundant and generally moderately to well preserved. Diatoms are represented by open-ocean forms not characteristic of coastal upwelling regions, except during the late Pliocene when upwelling forms are present in relatively high frequencies. The abundant occurrence of robust taxa, such as Coscinodiscus marginatus, Proboscia barboi (see "Site 1010" section, this chapter), Stephanopyxis spp., and Thalassiothrix spp., suggests an increase in coastal upwelling intensity during the late Pliocene through the beginning of the earliest Pleistocene.
The boundary between the late Quaternary Neodenticula seminae and Proboscia curvirostris Zones (NPD 12/11) is indicated by the LO of P. curvirostris in Samples 167-1020B-5H-CC and 167-1020C-6H-CC, respectively. The biostratigraphic interval through Zone NPD 12 and the uppermost part of Zone NPD 11 corresponds well to positive inclinations above 78 mbsf in Holes 1020B and 1020C, representing Chron C1n (Brunhes).
The LO of Actinocyclus oculatus in Samples 167-1020B-14H-CC and 167-1020C-15H-CC marks the boundary between the P. curvirostris and A. oculatus Zones (NPD 11/10) in these holes. This boundary is estimated to be between 1.01 and 1.46 Ma in the subarctic region as its LCO by Barron and Gladenkov (1995). The FO of P. curvirostris with an age of 1.58 Ma is recognized between core-catcher samples 14H-CC and 15H-CC in both Holes 1020B and 1020C. The normal-polarity Jaramillo Subchron (C1r.1n, 0.99-1.07 Ma) at about 89 to 98 mbsf in Hole 1020B supports an age older than 1 Ma for the LO of A. oculatus at Site 1020.
Koizumi (1992) proposed that the LO of Neodenticula koizumii clearly defines the base of the A. oculatus Zone (NPD 10) and the top of the underlying N. koizumii Zone (NPD 9). This latest Pliocene event at ~2.0 Ma falls between Samples 167-1020B-18H-CC and 19X-CC, and between Samples 167-1020C-19X-CC and 20X-CC, respectively. The occurrence of Fragilariopsis doliola in Samples 167-1020B-18H-CC and 167-1020C-19X-CC supports the recognition of the NPD 10/9 boundary, because the FO of that species has an estimated age of 1.9 Ma in the eastern equatorial Pacific (Baldauf and Iwai, 1995). Continuous common occurrences of Neodenticula kamtschatica, which verify the underlying N. koizumii-N. kamtschatica Zone (NPD 8), were not found at Site 1020. The LO of T. convexa, dated at 2.35 Ma, is assignable in Samples 167-1020B-25X-CC and 167-1020C-23X-CC, just above the barren interval.
Site 1021 is the deep-water drill site of the Gorda Transect. It is located about 100 km south of the Mendocino Fracture Zone on 29.6-Ma crust in 4213-m-deep water. Four holes were cored at Site 1021 to a maximum depth of 310.1 mbsf, recovering a well-dated, apparently continuous interval of uppermost middle Miocene to Quaternary (12.9-0 Ma) sediments. Dominant lithologies are clay, clay with nannofossils, nannofossil ooze, clayey diatomite, and diatom clay mixed sediment. The upper part of the sequence is characterized by a dominance of clay and calcareous nannofossils, whereas diatoms form the main biogenic component in the lower part.
The sedimentation rate for the last 10 m.y. was remarkably constant at ~30 m/m.y., but it was lower during the late middle Miocene (10-12.9 Ma). A complete magnetostratigraphy was determined in Holes 1021B and 1021C. All chrons from the Brunhes (C1n) to the onset of C3n.4n (Thvera subchron) at 5.23 Ma could be identified in the upper 160 mbsf. The microfossil groups are dominated by cool, high-latitude elements from the latest Miocene through the Quaternary. However, diatom assemblages are typical of hemipelagic sedimentary sequences of temperate middle latitudes, and the standard North Pacific diatom zonation has been adapted for use at Hole 1021B (Table 12). Both diatoms and radiolarians indicate that upwelling was strong during the middle through late Miocene.
Diatoms are generally common to abundant and moderately to well preserved in the middle and early late Miocene. In the Quaternary, lower Pliocene, and uppermost Miocene, diatoms are mostly rare and poorly preserved as a result of dissolution. Reworked forms and neritic assemblages are not important at this site. Sparse occurrences of subtropical taxa including Actinocyclus ellipticus, Azpeitia nodulifera, Crucidenticula nicobarica, C. punctata, and Hemidiscus cuneiformis suggest the influence of relatively warmer waters during the middle and early late Miocene.
A poorly preserved interval from 55.5 to 84.0 mbsf is generally correlative with the late Pliocene zones from the Neodenticula koizumii Zone (NPD 9) to the underlying N. koizumii-Neodenticula kamtschatica Zone (NPD 8), based on the presence of N. kamtschatica, Thalassiosira convexa, Neodenticula sp. A of Akiba and Yanagisawa (1986), and consistent N. koizumii. A couple of samples from 167-1021-14H-CC and 15H-CC (131.5-141.0 mbsf) are assigned to the N. kamtschatica Zone (NPD 7B), based on the presence of rare N. kamtschatica and Thalassiosira oestrupii.
Characteristic early Pliocene subarctic assemblages dominated by N. kamtschatica and Coscinodiscus marginatus are missing at Site 1021 because of the warm-temperate conditions. Thus, it is inferred that an unzoned interval between Samples 167-1021B-16H-CC and 23X-2, 20-22 cm, probably including the Miocene/Pliocene boundary, is equivalent in age to the lower part the N. kamtschatica Zone (NPD 7B) through the upper part of the Rouxia californica Zone (NPD 7A). Samples 167-1021B-23X-5, 20-22 cm, and 23X-CC are tentatively placed in Zone NPD 7A.
A complete sequence of diatom zones from the late Miocene Thalassionema schraderi Zone (NPD 6B) to the middle Miocene Denticulopsis praedimorpha Zone (NPD 5B) was penetrated in the lower half part (>214 mbsf) of Hole 1021B. The LCO of T. schraderi in Sample 167-1021B-24X-2, 20-22 cm, marks the boundary between the NPD 7A and the underlying the T. schraderi Zone (NPD 6B). From Zones NPD 7A through 6B, both characteristics of R. californica and T. schraderi, occur importunately but not so abundantly. The LCO of D. simonsenii is assigned to between Samples 167-1021B-26X-2, 92-94 cm, and 26X-5, 93-95 cm, where the NPD 6B/6A boundary is obviously placed. The LO of D. katayamae is not determined clearly because of an interpolation of poorly preserved horizon.
Assemblages in Samples 167-1021-27X-5, 12-14 cm, through 28X-CC (248.9-262.1 mbsf) containing Denticulopsis dimorpha are assigned to the early late Miocene D. dimorpha Zone (NPD 5D). Within Zone NPD 5D, Sample 167-1021B-27X-CC records the FO of D. katayamae, and Sample 167-1021B-28X-CC contains the FO of T. schraderi. The consistent occurrence of D. praedimorpha in Samples 167-1021B-31X-CC through 33X-CC corresponds to the D. praedimorpha Zone (NPD 5B) and indicates that the top of the zone is established by the LCO (LO) of that species at 290.9 mbsf.
Between Zones NPD 5D and 5B, the Thalassiosira yabei Zone (NPD 5C) lies between Samples 167-1021B-29X-2, 6-8 cm, and 167-1021B-31X-5, 20-22 cm. Although D. praedimorpha persists until the lower half of Zone NPD 5C, an evolutionary lineage from D. praedimorpha to D. dimorpha does not occur as in the mid-latitude northwestern Pacific. Crucidenticula nicobarica occurs in the bottom Samples 167-1021B-33X-5, 15-17 cm, and 33X-CC, and in addition D. praedimorpha still remains commonly, which surely indicates that the base of Hole 1021B is settled in age between 12.5 and 12.9 Ma (Barron and Gladenkov, 1995) or between 12.7-12.8 and 12.9 Ma (Yanagisawa and Akiba, 1998).
Specimens of Thalassiothrix, including T. longissima, are especially abundant and large in middle to late Miocene assemblages, indicating high oceanic productivity during this time. Simultaneously, Coscinodiscus marginatus and Proboscia barboi exhibit common-to-abundant frequencies, showing evidence for intense upwelling conditions (see "Site 1010" section, this chapter). In contrast, specimens of Denticulopsis are mainly represented by small size compared with those in diatom assemblages from the northwest Pacific, but coastal environments represented by a large quantity of Stephanopyxis species are never indicated at Site 1021.
Two periods of intense upwelling conditions are remarkable during the latest Miocene through the late middle Miocene. One corresponds to Zone NPD 7A down to the uppermost of Zone NPD 5C, and the other represents almost the entire Zone NPD 5B in the basal part of Hole 1021B. An interval of poorly preserved assemblages, probably resulting from strong dissolution, lies between the upwelling intervals. Such genera as Denticulopsis and Neodenticula, characteristic of subarctic oceanic environments, are small or scarce throughout most of the sequence, suggesting weak and episodic southward extension of the California Current at this location.
Site 1022 is situated on the continental slope just south of the Mendocino Fracture Zone about 90 km from Cape Mendocino. The drill site is located at a water depth of 1926 mbsl on a sliver of continental crust that may have been transferred to the Pacific Plate during the Oligocene. Three holes were cored at Site 1022, to a maximum depth of 387.8 mbsf, recovering an interval of Quaternary to late/early Pliocene age.
The Quaternary is represented by only a very thin (<1 m) veneer of sediments overlying the upper Pliocene. Both diatoms and radiolarians show evidence of strong upwelling throughout the Pliocene at Site 1022. A diagenetic boundary occurs at 360 mbsf, where diatomite is transformed into interbedded siliceous mudstone and chert. Low magnetic intensities and a drilling-induced overprint precluded the establishment of a magnetic chronostratigraphy.
Diatoms are generally common to abundant and poorly to moderately well preserved throughout the sequence. Pliocene diatom assemblages recovered from Holes 1022A and 1022C are dominated by cool temperate to subarctic taxa, with subtropical elements very seldom. Littoral forms and reworked assemblages are not significant at this site.
The standard North Pacific diatom zonation is difficult to use at Site 1022 despite the relatively northern locality at 40°N (Table 13). Whereas the upper Pliocene sequence at Site 1022 is the most diatomaceous of all Leg 167 sites, such diagnostic markers as Neodenticula seminae, Neodenticula koizumii, and Neodenticula kamtschatica are consistently trace to rare.
The late Pliocene zonal boundary between Zones NPD 9 and NPD 8 is difficult to place because of very rare occurrences of N. kamtschatica. The boundary is tentatively placed at the LO (= LCO) of N. kamtschatica between Samples 167-1022A-9H-CC and 10H-CC. The LO of T. convexa is assigned to Sample 167-1022A-5H-CC at 42.5 mbsf within the middle part of Zone NPD 9. The first occurrences of N. seminae and N. sp. A sensu Akiba and Yanagisawa are assigned to Sample 167-1022A-7H-CC at 61.5 mbsf and 8H-CC at 71.0 mbsf, respectively, just above the LO of N. kamtschatica at 90.0 mbsf. The FO of N. koizumii (top of Zone NPD 7), which is close to the lower/upper Pliocene boundary, occurs between Samples 167-1022A-15H-CC and 16H-CC.
Diatom species characteristic of upwelling conditions are sometimes dominant throughout the sequence, exhibiting clear oscillations associated with oceanic/coastal environmental changes. Radiolarians indicate a prevalence of strong coastal upwelling, whereas the diatoms reflect oscillations between strong coastal upwelling and occasional oceanic upwelling. The former is characterized by abundant, large Coscinodiscus marginatus and Stephanopyxis species; on the other hand, the latter is marked by abundant Thalassiothrix longissima and Proboscia barboi.
Hole 1022A consists of 166 m of upper Pliocene through possible uppermost lower Pliocene sediments. Continuous occurrences of N. koizumii in Hole 1022A prove the topmost core-catcher sample to be older than 2.0 Ma, and the lowest sample to be just older than 3.5-3.9 Ma. Co-occurrences of N. kamtschatica, Nitzschia reinholdii, and Thalassiosira oestrupii through Samples 167-1022C-31X-CC to 38X-CC support that the lowermost sample is younger than 5.49 Ma certainly. On board the JOIDES Resolution, planktonic foraminifers indicate that the top of the sequence is >2.25 Ma and that the base of Hole 1022A is older than 3.3 Ma. Calcareous nannofossils also provide data on the age of the base of Hole 1022A to be <3.8 Ma. Most of the microfossil groups well constrain the total age range of the sedimentary sequence recovered from Site 1022.