The diatom zonation described in Yanagisawa and Akiba (1998) was used along the California margin. Table 2 and Table 3 and Figure 2 summarize the lower Miocene to Quaternary diatom zonation and zonal datum levels used to date samples from Leg 167. Table 14 and Table 15 epitomize the stratigraphic and areal occurrences and the chronology of important diatom datum levels in a south-to-north transect from Sites 1010 to 1022 along the California margin. Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, and Figure 11 give the stratigraphic distribution of environmental characteristics in Leg 167, and Figure 12 summarizes diatom zonal data to correlate among all holes along the north-to-south direction.
Based on available synthesis studies (Koizumi, 1985; Akiba, 1986; Barron, 1992a; Yanagisawa and Akiba, 1998), the following Miocene datum levels appear to be essentially isochronous and widely applicable in the North Pacific through Leg 167: the FCO of Denticulopsis simonsenii (13.1 Ma), the FO of D. praedimorpha (12.9 Ma), the LCO of D. praedimorpha (11.5 Ma), the FO of D. dimorpha (9.9 Ma), the FO of Thalassionema schraderi (9.5 Ma), the LO of D. dimorpha (9.16 Ma), the LCO of D. simonsenii (8.6 Ma), the FCO of T. schraderi (8.4 Ma), the LCO of T. schraderi (7.6 Ma), the LCO of Rouxia californica (6.65 Ma), the FCO of Neodenticula kamtschatica (6.4 Ma), and the FO of T. oestrupii (5.49 Ma).
All these datum levels have been recognized to be of the essence of various North Pacific diatom zonations (Koizumi, 1992; Barron and Gladenkov, 1995). It should be mentioned, however, that one of the North Pacific diagnostic species, Neodenticula kamtschatica, is effectively excluded from California waters, and so the practical use of the N. kamtschatica Zone (NPD 7) is almost not allowed for the California region sediments. According to Barron (1989, 1992a), the FO of N. kamtschatica is delayed in California (~5.3 Ma), whereas Rouxia californica has an earlier LO (~6.5 Ma).
The FO of Probosia barboi (12.3 Ma) near the Zone NPD 5A/5B boundary and the LO of Denticulopsis katayamae (8.5 Ma) immediately below the top of Zone NPD 6A seem to be isochronous among Sites 1010, 1011, and 1021, but these events of cold-water taxa are slightly earlier in the North Pacific. Likewise, some of the FOs and LOs of warm-water diatom taxa appear to modulate across latitude in the North Pacific. Particularly, the LO of Crucidenticula nicobarica (12.5 Ma), the FO of Hemidiscus cuneiformis (11.5 Ma), the FO of Nitzschia fossilis (8.7 Ma), and the FO of N. reinholdii (7.4-7.5 Ma) gave some indication of their biostratigraphic uncertainty, but confirmation of their diachroneity must await magnetostratigraphic studies across middle latitudes in the North Pacific.
At Site 1010, warm-water diatoms such as Actinocyclus ellipticus, Annellus californicus, and Craspedodiscus coscinodiscus are relatively consistent in an interval from Subzone NPD 4Bb through the lower part of Zone NPD 5B. The restricted few-to-common occurrences of Azpeitia nodulifera within this interval support the possibility that warming conditions prevailed during this period (~13.1-12.0 Ma). At Site 1021 off northern California, the consistent presence of Actinocyclus ellipticus f. javanica, Azpeitia nodulifera, and Crucidenticula punctata within the correlative interval in Zone NPD 5B also support a relative warming trend compared to younger intervals of the Miocene deposits. At Site 1021 the relative warm interval (~12.8-12.0 Ma) is separated from the late Miocene cool interval (~7-10 Ma) by a dissolution interval between Cores 167-1021B-29X and 167-1021B-31X.
Prominent dissolution intervals similar to Hole 1021B occur in Subzone c of the Denticulopsis hustedtii—D. lauta Zone of Barron (1981) at that time, which is almost equivalent to Zone NPD 5C, in both DSDP Leg 63, Hole 470 at 28°54.46´N and Hole 472 at 23°00.35´N off Baja California. Preservation in the diatomaceous middle Miocene section is generally good to moderate, with the exception of DSDP Leg 63 Cores 470-13, and 472-9 through 472-10, respectively, where poorly preserved diatoms are present. As for the water depth, there is a similarity among the three holes, which are located at deeper seafloor >3500 mbsl; that is, Hole 470 at 3549 mbsl, Hole 472 at 3831 mbsl, and Hole 1021B at 4213 mbsl, respectively. Pronounced diatom dissolution is not due to silica diagenesis (Barron, 1981), but it is interpreted to be evidence of the onset of deep-to-intermediate water exchange along the California margin caused by slackening of the deep water circulation.
The FO of Nitzschia reinholdii is a secondary stratigraphic marker that is useful in the low-latitude Pacific. Barron (1985a, 1992a) assigned an age of 7.3 Ma for this event, which ought to fall within Zone NPD 7A. From Sites 1016, 1021, and 1022, the sporadic distribution of this species was observed above from Zone NPD 7A, so little can be said about its first occurrence.
The FO of the T. oestrupii group was observed in Holes 1016A and 1021B. Stratigraphic usefulness of this event was recognized as a secondary zonal marker in the North Pacific. Barron (1981) actually defined a Thalassiosira oestrupii Zone, whose base is defined by its FO, estimated at 5.49 Ma. This zone was used for the low- to mid-latitude Pacific, but it is difficult to make the zone up for a standard piece of the North Pacific zonation.
The following subtropical warm-water, age-diagnostic species are scarce in Leg 167 sediments: Fragilariopsis doliola, Nitzschia reinholdii, Rhizosolenia praebergonii, Nitzschia jouseae, and Thalassiosira miocenica. Therefore, the subtropical zonation of Baldauf and Iwai (1995) could not be applied to the Pliocene through Pleistocene sediments recovered from the California margin.
Not surprisingly, continuous common occurrences of Neodenticula kamtschatica, which characterize the diatom assemblages from topmost Miocene through Pliocene duration, were not found at most sites of Leg 167, according to expectation by previous studies of Barron and Baldauf (1986) and Barron (1989, 1992b). However, the FO of Neodenticula koizumii (3.53-3.95 Ma), the FO of Neodenticula seminae (2.68 Ma), the LO of N. kamtschatica (2.61-2.68 Ma), the LO of Thalassiosira convexa (2.35 Ma), the LO of N. koizumii (2.0 Ma), the FO of Fragilariopsis doliola (2 Ma), the FO of Proboscia curvirostris (1.5 Ma), the LO of Actinocyclus oculatus (1.01-1.46 Ma), the FO of Rhizosolenia matuyamai (0.99-1.14 Ma), the LO of R. matuyamai (0.91-1.06 Ma), and the LO of P. curvirostris (0.30 Ma) appear to be nearly isochronous along the California margin in Leg 167.
As recognized by Koizumi and Tanimura (1985), the FO of Neodenticula koizumii is diachronous across latitude ranging from about 3.95-3.53 Ma. Barron and Gladenkov (1995) regret the widespread use of this datum level in North Pacific diatom stratigraphy where it marks the top of the N. kamtschatica Zone (NPD 7) and the base of the overlying N. koizumii-N. kamtschatica Zone (NPD 8). They hoped accordingly that the FO of Actinocyclus oculatus (3.6-4.0 Ma) might prove to be a more reliable stratigraphic marker than the FO of N. koizumii in the middle part of the Pliocene north of about 40°N.
Valves of A. oculatus are easier to identify at low magnification in the light microscope because of their commonly blue interference colors and distinctive areolar pattern. Similarly, the more robust nature of the valves of A. oculatus, compared with those of N. koizumii, would suggest that it should be less susceptible to dissolution. However, A. oculatus is very sparse and sporadic in Leg 167 sediments, and probably is more common in pelagic regions of the North Pacific and the Bering Sea.
Moreover, Yanagisawa and Akiba (1998) argued that the morphology of N. koizumii has given rise to a taxonomic problem between its LO and the FO of Neodenticula seminae, defining the top of the N. koizumii Zone (NPD 9) and the base of the overlying A. oculatus Zone (NPD 10). Koizumi's (1992) proposal to use the LO of N. koizumii to mark the top of the N. koizumii Zone (NPD 9) appears to be justified in place of the LO of warm-water taxon Fragilariopsis doliola. This zonal boundary falls just below the Pliocene/Pleistocene boundary in the mid- to high-latitude North Pacific.
The FO of Fragilariopsis doliola is observed from Zone NPD 10 at Sites 1018 and 1020. This event in the low-latitude Pacific has been calibrated with the middle of the Olduvai Subchron and has an age of 1.90 Ma (Baldauf and Iwai, 1995). In addition, it is diachronous between the low and middle latitudes, with age estimates ranging from 1.8 to 2.0 Ma in the northwest Pacific (Koizumi and Tanimura, 1985). The FO of F. doliola is slightly younger than the base of Zone NPD 10 estimated at 1.9 Ma in Leg 167.
It also should be emphasized that N. kamtschatica can be very sparse and sporadic in Leg 167 sediments, so that recognition of its LO at about 2.6 Ma may be very difficult to verify the boundary between Zones NPD 8/9. The LO of Thalassiosira convexa correlates to an interval directly above Chron 2An and has an estimated age of 2.35 Ma in the equatorial Pacific (Baldauf and Iwai, 1995). This event also was recognized at Sites 1014, 1018, 1020, and 1022 along the California margin, but a poor magnetic record prohibited age estimates.
All of these results uphold the insistence of Baldauf and Iwai (1995) and Barron and Gladenkov (1995) that quantitative studies accompanied by paleomagnetostratigraphy are necessary to adequately resolve age estimates of the Pliocene and Pleistocene diatom datum levels in the North Pacific.
In present-day waters off California, diatoms owe their abundance to coastal upwelling of nutrient-rich waters, which is driven by the persistent southward winds that are associated with the southward-flowing California Current. Slackening of the California Current during El Niño periods results in a diminished southerly current, decreased upwelling, and diminished diatom production. Because the preservation and abundance of diatoms in sediments is directly related to their abundance in the overlying surface waters, a decline in spring diatom productivity caused by a slackening of the California Current would result in a decreased diatom sedimentation rate and in a decline in diatom preservation in seafloor sediments. Consequently, we can trust that intervals of poor diatom preservation in pelagic sediments off California would correspond to a slackening of the southward flow of the California Current.
An interval of poor diatom preservation, containing only rare, poorly preserved diatoms, is present throughout much of the latest Miocene through Pliocene prior to Zone NPD 10 (2.6 Ma) at Leg 167 sites off California (Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 12). Diatom stratigraphy reveals the diatomaceous interval from middle to late Miocene, but in the overlying units diatoms quickly disappear upsection as the terrigenous component increases at typically 7.0 Ma. Similarly, the early Pliocene diatom record of California is very poorly known due to increased terrigenous dilution in onshore sections, the widespread presence of hiatuses or condensed intervals in both onshore and offshore sections, and the pervasive appearance of dissolution in sequences south of about 40°N.
Barron (1981, 1992b) interpreted that this diatom-poor interval, mainly a dissolution interval, coincides with a period of high-latitude warming or deglaciation and it is taken as evidence of decreased southerly flow of the California Current and a reduction in upwelling off California during the middle part of the Pliocene. He also inferred that the intervals of diatom dissolution correspond to August sea-surface temperatures in excess of about 17°C.
This interval of poor diatom preservation is obviously detected at Sites 1010, 1011, 1014, 1018, and 1021, indicating stagnation of the mid-latitude surface waters (Fig. 3, Fig. 4, Fig. 12). At Sites 1010 and 1011 off southern California, poor diatom preservation begins between Zones NPD 6B and 7A, where it is estimated at ~7 Ma. Likewise, at the northern Site 1021 the beginning of the poor diatom preservation can be identified between Zones NPD 7A and 7B. Presumably, both rapid progress of cooling, symbolized by a global fall in sea level, and successive warming caused a decline in diatom abundance in the surface waters above Sites 1010, 1011, and 1021 and resulted in poor diatom preservation.
Dissolution of diatom assemblages is typical in the early Pliocene and early part of the late Pliocene, indicating a marked decline in the production of diatoms in surface waters. The onset of diatom dissolution at Sites 1021 off northern California at the Miocene/Pliocene boundary perhaps within Zone NPD 7B coincides with the start of a global fall in sea level in the period of major cooling.
The northward flow of the warm waters along the western Pacific rim might be so extremely weakened that the clockwise circulation pattern passing across the mid-latitude North Pacific declines abruptly at the same time. Relatively warmer waters originating from equatorial waters accumulated off central North America accordingly, and these stagnant waters also screened out the California margin from a direct supply of cold waters from the counterclockwise circulation in the Gulf of Alaska. The southward flow of the California Current probably slackened at this time, resulting in reduced coastal upwelling and diatom blooms in offshore areas.
Nevertheless, abundant diatoms persisted in the deeper Site 1016 (distance of 148 km from shore, water depth 3846 m) in the central California region as well as at the more northerly Site 1022 (distance of 87 km from shore, water depth 1927 m). A reduction in upwelling off California during the middle part of the Pliocene had less effect on Sites 1016 and 1022 (Fig. 7, Fig. 9, Fig. 11, Fig. 12).
The onset of poor diatom preservation at nearshore southern Sites 1010 and 1011 in the latest Miocene may signal a generalized narrowing of the entire California Current system rather than further high-latitude warming. Diatom production probably continued in these coastal waters, but diatom sedimentation was almost completely masked by the clay-rich clastics from the California Borderland (Fig. 7).
Well-preserved diatoms did not return to Sites 1016, 1018, 1020, and 1022 until 2.6 Ma, almost corresponding to the NPD 8/9 boundary. At Site 1018 off central California, the interval of poor diatom preservation immediately before 2.6 Ma may indicate a generalized narrowing of the California Current rather than further high-latitude warming, because diatoms also disappear from Sites 1010 and 1011 off southern California at this same time and do not appear again in younger sediments (Fig. 3, Fig. 4, Fig. 5, Fig. 12). Off central to northern California common-to-abundant distribution of such dissolution-resistant taxa as Coscinodiscus marginatus and Stephanopyxis spp. were consistently recorded at Sites 1018 through 1022 during the late Pliocene Zone NPD 8 to the Pleistocene Zone NPD 11, and especially the late Pliocene duration from Zones NPD 8 to NPD 9 (Fig. 8, Fig. 10).
The warm waters (Kuroshio Current) come into collision toward the front of the cold-water mass (Oyashio Current) with low rapidity in the northwest Pacific off Japan, and then change their flow direction to the east. In contrast, in the northeastern Pacific somewhat warm waters crossing the middle latitudes flow abreast with the cold waters driven from the Gulf of Alaska, and then drifted toward the south from offshore Oregon. The weaker the power of traversing waters, the less able the California Current is to flow to the south. Accordingly, paleoceanographic provincialism expands greatly between the California margin and the high-latitude northeastern Pacific. In present-day oceanography off California, current waters come from the mid-latitude North Pacific, except in summer, when fleet waters originating from the Gulf of Alaska directly invade along the California margin.
Reduced upwelling with resultant decreased diatom productivity is a possible explanation for the scarcity of diatoms within the middle part of the Pliocene Epoch off California. The interval of poor diatom preservation fairly corresponds to the duration from Event C to Event D of Barron (1998). He explained that the Event C at about 4.5 Ma is characterized by the onset of a period of sustained high-latitude warming that lasted at least l m.y. Diatom sedimentation rates increased at higher latitudes of the northwest Pacific, whereas they declined off northeast Japan and were apparently waning off California. During this climatically warm period, large quantities of relatively warm, saline surface waters penetrated further to the north (Barron, 1995, 1998).
Sancetta and Silvestri (1986) maintained that the modern subarctic water mass did not exist in the western Pacific, but a broad transition zone between warmer and cooler waters was present in the North Pacific, with a northern margin north of 48°N and a southern margin south of 41°N. Such a broad transition zone would have resulted in a northward spread of relatively warm, salty surface waters, which would lead to enhanced evaporation at the surface, to decreased vertical stratification of the water mass, and to an increased upward diffusion of nutrient-rich deep waters, and accordingly fueled diatom production. Based on the diatom paleoclimatic ratios from the middle part of the Pliocene, Barron (1995) reconstructs warmer sea-surface temperatures distributed in the broad transition zone of the northwest Pacific.
On the other hand, the apparent decline in diatom productivity in middle latitude regions, such as the California margin and the coasts of Japan, indicates a reduced pole-to-equator thermal gradient resulting in a slackening of offshore winds and reduced upwelling. Based on the modern distribution of diatoms, it appears that the offshore intervals of diatom scarcity coincided with August sea-surface temperature of more than 17°C (Barron, 1992b). The onset of a period of climatically warmer high-latitude paleotemperatures and paleoceanographic circulation changes appears to link with a shoaling of the Isthmus of Panama, which may have induced major changes in diatom sedimentation in the Pacific at about 4.5 Ma.
At Event D of Barron (1998) after 2.7 Ma, diatom accumulation rates sharply declined at the higher latitudes of the North Pacific, coincident with a major increase in ice-rafted detritus and the onset of Northern Hemisphere glaciation. Increased vertical stratification of water masses resulted in a slackening in the upwelling of nutrient-rich deep waters in the northwest Pacific, and high diatom productivity probably shifted more toward the coasts of Asia and North America. Increased offshore winds appear to have enhanced upwelling and renewed diatom productivity off the coast of California at about the same time (Barron, 1981, 1992b). A true increase in diatom accumulation off California at about 2.7 Ma is suggested by the replacement of dissolved diatoms by well-preserved diatom assemblages at this time at DSDP Sites 467 and 469 as well as ODP Sites 1016, 1018, 1020, and 1022. Probably, this preservational change reflects enhanced diatom productivity due to an increase in zonal winds driving the upwelling along the California margin (Barron, 1981, l992b, 1998).
Because the Isthmus of Panama apparently had completely emerged by 2.7 Ma, Event D in the high-latitude North Pacific was not directly related to the closure of the Central American Seaway. Rather, Barron (1998) indicates that major reorganization of surface- and intermediate-water masses at about 2.7 Ma would be the presumable trigger of contemporaneous changes in diatom sedimentation patterns that are documented in the North Pacific.