The Neogene sedimentary record along the California margin is somewhat familiar because the outlines of the Neogene history have been assembled from earlier drilling (Deep Sea Drilling Project [DSDP] Legs 5, 18, and 63; Barron,1989, 1998). The discontinuous nature of the DSDP drilling recovery, the scattered positions of the older drilling sites, and the lack of good stratigraphic control meant that detail was lacking prior to Leg 167 drilling and that much of the structure within the sedimentary record could not be studied. Much of the potential usefulness of the Leg 167 records has yet to be exploited because of Pleistocene research had the priority in initial postcruise studies. So far, most of the Neogene research has focused upon development of better regional stratigraphic control.
We describe only a reconnaissance of major paleoceanographic developments along the margin, but even at this early stage we note that major changes in paleoceanographic conditions since 14 Ma have relationships, unsurprisingly, to exposed sedimentary sections of equivalent age in coastal California; for example, the Miocene Monterey and Sisquoc formations. We also identified a major CaCO3 depositional event in the late Pliocene that abruptly ended or strongly diminished in strength at 2.6 Ma with the beginning of Northern Hemisphere glaciation (Ravelo et al., 1997). Fine-scale structure of these events still needs to be measured and the implications for paleoceanography, the biogeochemical cycle, and climate need to be understood.
One of the primary goals of Leg 167 was to collect magnetostratigraphic records of the Quaternary and Neogene that could be used to provide a good chronostratigraphic framework to evaluate the isochroneity of biostratigraphic events along the California margin. Prior to Leg 167, magnetostratigraphic studies in the middle-latitude northeastern Pacific were limited to relatively fragmentary onshore records (e.g., Madrid et al., 1986; Omarzai et al., 1993).
Leg 167 provided excellent magnetostratigraphic records of the last 6.3 m.y. The longest one is from the composite section of Holes 1010C and 1010E (Hayashida et al., 1999), which yielded a high fidelity record from C1n (Brunhes) to C3A.n2 (latest Miocene; Table 5; Fig. 33). The magnetostratigraphic reversal record from Site 1021 is also lengthy, extending from C1n (Brunhes) to C3n.4n (5.23 Ma; earliest Pliocene). The magnetostratigraphic records at other Leg 167 sites—Sites 1012, 1013, and 1014—are excellent for the Quaternary, but unfortunately they do not extend back beyond the Olduvai (1.195 Ma; see Table 1 of Lyle, Koizumi, Richter, et al., 1997).
The study of Fornaciari (Chap. 1, this volume) represents the first real attempt since Bukry's (1981) study of DSDP Leg 63 to refine calcareous nannofossil biostratigraphy along the California margin. Fornaciari (Chap. 1, this volume) has evaluated the reliability of 50 calcareous nannofossil biostratigraphic events (biohorizons) by considering their mode of occurrence, ranking, and spacing. Four biohorizons in the Pleistocene are considered good and apparently synchronous (Table 5; Fig. 34): the last occurrence (LO) of Pseudoemiliania lacunosa, the LO and first occurrence (FO) of large Gephyrocapsa, and the FO of G. oceanica s.l. All these biohorizons can be recognized across the entire Leg 167 latitudinal transect in the California margin (Table 5; Fig. 33).
The Pliocene calcareous nannofossil assemblage is generally poorly diversified and badly preserved. The following six biohorizons are considered reliable in the Pliocene interval: the LO of Discoaster pentaradiatus, the LO of D. surculus, the LO of D. tamalis, the LO of Reticulofenestra pseudoumbilicus, the LO of Amaurolithus delicatus, and the paracme beginning (PB) of D. pentaradiatus (Table 5; Fig. 34).
The late Miocene calcareous nannofossil assemblages are distinctive, as discoasterids are generally rare to scarce because the area was affected by high productivity conditions that are unfavorable for discoasterids. In particular, the standard zonal marker species of Okada and Bukry (1980) are quite rare. Based on their occurrences at ODP Sites 1010, 1011, and 1021, ten late Miocene calcareous nannofossil datum levels appear to be useful for correlation (Table 6, Fig. 34, Fig. 35): the LO of D. quinqueramus/berggrenii, the FO of A. primus, the FO of D. quinqueramus/berggrenii, the LO of D. hamatus, the FO of D. bellus, the FO of Catinaster coalitus, the LO of Cyclicargolithus floridanus, the FO of D. kugleri, the LO of Calcidiscus premacintyrei, and the LO of Sphenolithus heteromorphus. Additional older middle and early Miocene calcareous nannofossil datum levels that have proven to be useful for correlation in the California area (Bukry, 1981; pers. comm., 1999) are included on Figure 35.
Previous work on diatom biostratigraphy along the California margin is summarized in Barron (1981, 1992a, 1992b). Correlation of diatom datum levels to magnetostratigraphy in the mid-latitude North Pacific has been poor, especially below the latest Pliocene. Maruyama (Chap. 3, this volume) identifies at least 31 middle Miocene through Pleistocene diatom datum levels (biohorizons) that allow precise correlation along the California Current from Sites 1010 to 1022. He identifies six biohorizons that appear to be nearly synchronous through the interval from Pliocene to Pleistocene sediments recovered from the California margin: the FO and LO of Proboscia curvirostris, the FO of Fragilariopsis doliolus, the LO of Thalassiosira convexa, the LO of Neodenticula kamtschatica, and the FO of Neodenticula seminae (Table 5, Fig. 33, Fig. 34). Within the northern region, the FOs of N. koizumii and Stephanopyxis dimorpha may also be useful for stratigraphic correlation (Table 5, Fig. 33, Fig. 34). Because of dissolution or poor preservation in the southern sites and the cool-water character of the northern sites, subtropical age-diagnostic species are scarce in the Pliocene and Pleistocene sediments recovered by Leg 167.
Maruyama (Chap. 3, this volume) identifies 11 biohorizons that are essentially synchronous through Miocene interval of Leg 167: FO of Thalassiosira oestrupii, the last common occurrence (LCO) of Rouxia californica, the first common occurrence (FCO) and LCO of Thalassionema schraderi, the FCO and LCO of Denticulopsis simonsenii, the FO and LO of D. dimorpha, and the FO and LCO of D. praedimorpha (Table 5, Table 6, Fig. 33, Fig. 34, Fig. 35). All these biohorizons have been recognized to be of the essence of various North Pacific diatom zonations (Koizumi, 1992; Barron and Gladenkov, 1995). Additional older middle and early Miocene diatom datum levels that have proven to be useful in the California region (Barron, 1992a) are included on Figure 35.
An interval of poor diatom preservation containing only rare and poorly preserved diatoms is present from the late Miocene (7.0 Ma) through the early Pliocene (2.6 Ma) (Maruyama, Chap. 3, this volume). Diatom stratigraphy reveals the diatomaceous interval from middle to late Miocene, but in the overlying units prior to 2.6 Ma diatoms quickly disappear due to the increase of terrigenous component. Maruyama (Chap. 3, this volume) interpreted the poor diatom preservation to be a result first of rapid cooling, which is also indicated by a global fall in sea level, followed by a warming that should have caused a decline in diatom abundance in the surface waters.
The planktonic foraminiferal assemblages are consistently dominated by few taxa. A total of seven biohorizons of evolutionary changes within the Neogloboquadrina plexus are broadly applicable in the interval from the late early Pliocene to the Quaternary (~3.5 Ma to present day) throughout the region (Kennett et al., Chap. 2, this volume): the LO and FO of Neogloboquadrina pachyderma A (dextral, inflated form), the FO of N. pachyderma B (sinistral, inflated form), the FO of N. pachyderma C, the LO of N. kagaensis, and the LO and FO of N. asanoi (Table 5, Fig. 33, Fig. 34). The FO of Globorotalia inflata (3.3 Ma) also appears to be useful for biostratigraphy. Older, biosiliceous-rich sediments in the sequences generally lack planktonic foraminifers.
No radiolarian studies beyond those reported in the Initial Reports volume (Lyle, Koizumi, Richter, et al., 1997) were completed for this volume.
We have observed that physical properties of sediments such as color are useful for high-resolution correlations in the Pleistocene section (Fig. 30). Profiles of physical properties will also prove useful to develop better correlations in the Neogene section as well (Fig. 36). The profiles have both megayear and high-frequency structure, which should be useful for basic correlations and for later high-resolution tuning. Figure 36 is a comparison of GRAPE bulk density and 450-500 nm reflectance time series from Sites 1010, 1021, and 1022. It illustrates the strong variability on the million-year scale. Episodes of high reflectance and high bulk density occur about every 2 m.y. at Sites 1010 and 1021. We believe that the profiles are controlled by CaCO3 variations, because CaCO3 ranges from 0% to 60% at Site 1021 and from about 0% to 80% at Site 1010 (Lyle, Koizumi, Richter, et al., 1997), and because CaCO3 is white throughout the visible spectrum. This hypothesis needs further confirmation, but would suggest that the high CaCO3 event in the mid-Pliocene discussed by Ravelo et al. (1997) is not unique to the time immediately prior to Northern Hemisphere glaciation. It also appears that high CaCO3 events were comparable in magnitude between the southern Site 1010 and the northern Site 1021 prior to ~8 Ma. After 8 Ma, north-south differences in CaCO3 content were enhanced, perhaps as the latitudinal thermal gradient increased. Further work is needed to better define the high-resolution structure of these Miocene and early Pliocene events and to better constrain their timing.
The multiple biostratigraphic events (biohorizons) along the California margin, after being placed within a chronological framework by integration with paleomagnetic stratigraphy, enables correlation between the Leg 167 drill sites. Comparison between drill sites identifies distinct biostratigraphic events in the California Current system which are placed within an absolute time frame.
During the middle Miocene through Quaternary paleoclimatic and paleoceanographic conditions varied considerably throughout the 29°N to 40°N latitudinal transect drilled during Leg 167. Subtropical and subarctic floras and faunas continually mixed along this zone with the result that standard microfossil zonations were not easily applicable (Fornaciari, Chap. 1, this volume; Maruyama, Chap. 3, this volume; Kennett et al., Chap. 2, this volume). For example, the North Pacific diatom zonation of Akiba (1986) and Yanagisawa and Akiba (1998) could not readily be applied in the latest Miocene through early late Pliocene (Maruyama, Chap. 3, this volume).
California margin sediments changed from biogenic silica oozes in the middle Miocene to essentially pure terrigenous clastics by the early Pliocene. The evolution is clearly seen in the opal records of Janecek (Chap. 16, this volume, Fig. 37) as well as the shipboard sediment descriptions. We hypothesize that the changes in biogenic silica mark events in nutrient availability to the California margin, and that the system gradually switches from high-nutrient surface waters in the latest middle Miocene to surface waters almost barren of nutrients for at least a short interval in the earliest Pliocene. We do not yet understand how the conditions evolved, or even exactly what conditions existed, and further study to understand the evolution of paleoproductivity at the California margin will prove fruitful to a broader understanding of biogeochemical cycles in the oceans and the paleoceanography of the North Pacific in the middle Miocene (Vincent and Berger, 1985).
The first big drop in biogenic opal burial occurs during the same time period as the late/middle Miocene CaCO3 crash around the isthmus of Panama (Lyle et al., 1995; Roth et al., 2000). The late/middle Miocene boundary appears to represent a time that thermohaline circulation reorganized, partly because of the initial closing of the Panama gateway, but partly because of intensification of North Atlantic Deep Water (Roth et al., 2000). The peak opal burial along the California margin (~8-9.5 Ma) is older than peak burial in the equatorial Pacific (~5-7 Ma, Farrell et al., 1995), hinting at an out-of-phase relationship between nutrient supply to the northeastern Pacific and the equatorial Pacific during the late Miocene.
The high opal intervals in both Sites 1010 and 1021 are also high CaCO3, at least at the coarse scale for which we now have data. Future work will be needed to determine whether at high resolution we will find an alternation between the plankton groups or fine-scale coherence.
At Sites 1010 and 1011 off Baja California and at Site 1021 off Northern California, the abundance of diatoms in the sediments declined during the latest Miocene to earliest Pliocene in a series of steps, beginning at about 7.5 Ma (base of NPD7) and continuing further at about 6.5 and 5.1 Ma (see Maruyama, Chap. 3, this volume; Janecek, Chap. 16, this volume; and smear-slide data in Lyle, Koizumi, Richter, et al., 1997). This offshore restriction in diatom productivity coincided with the progressive focusing of upwelling and biosiliceous productivity into the coastal regions (Isaacs, 1985; Barron, 1998).
The opal-rich sediments discovered offshore in Leg 167 correspond to the well-known diatom-rich sediments of the middle and late Miocene Monterey Formation of southern and central California. There are major differences, however. In the Monterey formation, opal MAR actually increases upsection but is hidden by high influxes of terrigenous debris (Isaacs, 1985). Opal MAR in the Santa Barbara area increases in a series of steps from 10 Ma, for example, reaching the highest MARs between 6 and 4 Ma in the Sisquoc Formation. Offshore we have noted something of a reverse pattern—highest opal deposition prior to 11 Ma, a low period around 10 Ma, and high deposition between 7.5 and 9 Ma, followed by step-like drops at about 7.5 and 5 Ma. It would appear that upwelling along the California margin evolved from a relatively broad feature (Site 1021 was more than 300 km offshore and is not by any stretch of the imagination within the coastal upwelling zone) in the middle and early late Miocene to a system concentrated on the coast in the latest Miocene and early Pliocene. We have no understanding why this happened, and follow-up studies, especially those that stratigraphically link the outcrops onshore to the Leg 167 drilling, should prove to be a fruitful new avenue of research.
At about 4.5 Ma, coincident with the onset of a mid-Pliocene period of reduced latitudinal thermal gradients, diatom productivity virtually disappeared in the offshore waters of southern California (Barron, 1981; Maruyama, Chap. 3, this volume; Site 1016) and declined considerably at northern Site 1021 (Janecek, Chap. 16, this volume). The interval between about 5 and 4 Ma also tends to be an interval almost barren of foraminifers and calcareous nannofossils (Lyle, Koizumi, Richter, et al., 1997; Fornaciari, Chap. 1, this volume; Kennett et al., Chap. 2, this volume). This hiatus was also observed in Sites 467, 469, and 470 from DSDP Leg 63 (Barron, 1989).
Diatom productivity reappeared in strength only during the late Pliocene (at about 2.7-2.4 Ma) after a period of major high-latitude cooling led to enhanced upwelling along the California coast (Barron, 1998; Maruyama, Chap. 3, this volume; Janecek, Chap. 16, this volume). Latest Pliocene and Quaternary diatom assemblages in California waters are more comparable to those of the higher latitude North Pacific, and the zonation of Akiba (1986) and Yanagisawa and Akiba (1998) is applicable.
In contrast, high production of CaCO3 appeared in the mid-Pliocene and dropped abruptly at 2.6 Ma (Ravelo et al., 1997), coincident to the major increase in ice rafting in the North Pacific associated with the formation of major Northern Hemisphere glaciers (Krissek, 1995; Haug et al., 1995; Maslin et al., 1995). Ravelo et al. (1997) have concluded that the cause of the late Pliocene CaCO3 event was high production, not low dissolution. There is a similarity in timing between the California margin event and the late Pliocene productivity pulse in the Alaska Gyre (Haug et al., 1995) that becomes less so to the south. Northern California margin sites (e.g., Sites 1020 and 1021) have a more abrupt termination at about 2.6 Ma. Southern California sites, in contrast, have a decrease in production at 2.6 Ma but continue to have high CaCO3 burial until the Pliocene/Pleistocene boundary.
It is interesting to note that the late Pliocene CaCO3 event is not unique along the California margin—events of similar intensity though not necessarily of similar duration can be found at roughly 5.2, 6.8, 8.8, 10, and 12-14 Ma (Fig. 36). The 5-Ma event may also be present in the North Pacific (Haug et al., 1995). It is uncertain why the North Pacific periodically becomes a major burial ground for CaCO3, and it is worthy of further study.
One of the least explored research topics by Leg 167 researchers is the linkage of paleoceanographic and sedimentologic evolution of the northeastern Pacific with tectonic developments around the Pacific Basin. These include teleconnections from such events as the closure of the Panama Gateway and the uplift of the Himalaya Mountains. They also include the regional tectonics of western North America, because the formation of the Basin and Range and the uplift of the Sierra Nevada and the Cascade Range have probably affected basin configurations, runoff to the Pacific Ocean, and the strength and position of the northeast Pacific High (see Fig. 3). We will briefly explore some important issues here in the hopes of encouraging further research into the tectonic linkages.
Three primary difficulties make it difficult to establish a definite link between a tectonic event (e.g., the rise of the Himalayas) and a major climate change. First and most important, there is rarely any good age control on the tectonic event so that it is usually difficult to prove conclusively that the tectonic and climate events happened at the same time. Second, tectonic events are long, and several important tectonic events may overlap. For example, the Panama Gateway closes during the same time interval in which the Himalayas are rising and the Indonesian Passage is closing. It is difficult to ascertain which event may be the critical factor for the climate change. Finally, climate may be sensitive not only to the final state but to some lesser threshold. For example, Lyle et al. (1995) have suggested that CaCO3 burial was severely inhibited in the eastern Pacific Ocean when the deep-water connection through the Panama gateway was severed near the middle/late Miocene boundary, or about 7 m.y. before the final closure of the isthmus (Coates and Obando, 1996). A carbonate crash occurred on the Caribbean side of the Panama gateway also, but reached its peak extent about 2 m.y. before the Pacific side (Roth et al., 2000). A "simple" gateway closure can leave behind a complex sedimentary response.
The best way to come to an understanding of linkages between tectonics and climate is to continue to document events recorded in marine sediments and to improve our understanding of the timing of the tectonic processes.
The primary teleconnections that could be important to the development of the California Current system are the rise of the Himalayas, which could strengthen the Aleutian Low in the Subarctic North Pacific as the mountains rose (Kutzbach et al., 1993b); the closure of the Panama gateway, by modifying deep-water flow to the North Pacific (Haug and Tiedemann, 1998; Driscoll and Haug, 1998; and references therein); and closure of the Indonesian passage near the late/middle Miocene boundary, which should have enhanced heat transport northward in the western Pacific (Kennett et al., 1985; Romine and Lombari, 1985).
The uplift of the Himalayas, like all major orogenies, is a complex and long-lasting event. Initial uplift may have begun in the latest Oligocene, but there were two major later stages of mountain building in the Miocene and Pliocene (7.5-9 Ma and 3.5-4 Ma; Rea et al., 1998, and references therein). There is possibly a link between these mountain building events and (1) the loss of biogenic silica from Leg 167 sections around 8 Ma or (2) the beginning of the Pliocene CaCO3 burial event on the California margin (~3.5-4 Ma; Ravelo et al., 1997). Any tie remains speculative without a specific hypothesis for the teleconnection between Himalayan uplift and productivity on the California margin, however.
There is little information on the closure of the Indonesian gateway, but oceanographic severance seems to have occurred in the period between 11.5 and 10 Ma, using modern age models with Romine and Lambari (1985) stratigraphy. Again this is enticingly close to a major decrease in biogenic silica content in the California margin sediments (~11.5 Ma; Fig. 37). However, this is also a period when it appears that the closure of the Panama Gateway cut off much of the deep-water exchange between the Caribbean and eastern Pacific Ocean (Lyle et al., 1995; Roth et al., 2000).
The closure of the Panama gateway is a favorite tectonic element to modify oceanographic conditions in both the Pacific and Atlantic Oceans in the period between the mid-Pliocene and the late/middle Miocene boundary. The timing of closure depends upon what one is tracing however. Closure times are about 9-11 Ma for deep-water exchange (Lyle et al., 1995), 8 Ma for a significant interoceanic biogeographic barrier (with a reopening around 6 Ma; Collins et al., 1996), 4.6-4.4 Ma for a surface-water barrier sufficient to make Caribbean surface water distinctive from Pacific surface water (Keigwin, 1982; Haug and Tiedemann, 1998, Billups et al., 1999), and 3.1-2.5 Ma for an uninterrupted land bridge and the initiation of mammal exchange between North and South America (Coates and Obando, 1996; Webb and Rancy, 1996).
Haug et al. (1998) have suggested that a strong teleconnection exists between the Panama gateway closure and North Pacific productivity and that this teleconnection caused the major Pliocene production event between 4.6 and 2.7 Ma. If there is a link, it should be shown in some manner along the California margin. Ravelo et al. (1997) have already pointed out the similarity between the mid-Pliocene carbonate burial event and the North Pacific production, because both end abruptly within the same interval. Further work to establish whether closure of the Panama gateway should produce these events should be a priority for late Neogene studies.
The California margin of North America has had a complex tectonic history throughout the Neogene beginning with first the subduction of the Farallon Ridge in the Oligocene (Atwater and Stock, 1998; Bohannon and Parsons, 1995; and references in both papers) followed by the middle to late Miocene development of the Basin and Range province, a change in relative plate motions between the Pacific and North American plates in the late Miocene around 8 Ma (Atwater and Stock, 1998), the beginning of San Andreas fault motion around 6 Ma, and the uplift of the Sierra Nevada mountains since 10 Ma (1 km elevation at 10 Ma, 2 km elevation at 3 Ma, and 3 km elevation today; Huber, 1981). There are other, more speculative tectonic connections that might have affected the paleoceanographic evolution of the region. The most prominent of these is the possibility that the Mendocino Fracture Zone south of the Gorda spreading center may have been above sea level until about 5.5 Ma (Fisk et al., 1993). If the Mendocino Fracture Zone had been above sea level, it would have significantly altered California Current flow in the Miocene.
The Neogene records from Leg 167 potentially contain pertinent data on how local basins have responded to the changing tectonic regime, as well as how the regional tectonics have affected the climate and changed California Current circulation. Much if not all of this information has yet to be seriously studied. We note that a low-deposition event occurs throughout the California Borderlands (Sites 1011, 1012, and 1014) from the early Pliocene to late Miocene (>4 to perhaps 7 Ma; Fig. 38). This event is not found offshore (Fig. 39; Sites 1010, 1016, and 1021) and therefore appears to be regional to the California Borderlands. This may be linked to the initiation of the San Andreas fault system at about 6 Ma and a changing stress regime in the California Borderlands during the transition from movement on offshore faults to movement inshore of the borderlands region. More careful follow-up studies are needed to explore this possibility.
One would expect that uplift of the Sierra Nevada should have trapped more precipitation in western California and increased river gradients on the western slope of the Sierra Nevada Mountains. Both effects should cause an increased sediment load out of the mountain range from the Pliocene to the Holocene. Site 1018, because it lies just south of the mouth of the Sacramento River drainage, should be the best place to monitor these changes. What we observe at the drill site shows no real trend since the late Pliocene, however. Instead, sedimentation rates average slightly less than 120 m/m.y. in the period from 2.4 to 0.46 Ma and jump to about 200 m/m.y. in the period from 0 to 0.46 Ma (Lyle, Koizumi, Richter, et al., 1997; Andreasen et al., Chap. 8, this volume). The upper sediment section has more coarse clastic debris and appears to have a higher deposition rate of terrestrial detritus. The major change in sedimentation at around 0.5 Ma may reflect the capture of sediments in the Great Valley of California prior to 0.5 Ma and their escape afterwards. Sarna-Wojcicki (1995) has suggested that a mid-Pleistocene lake ("Lake Clyde") existed up until about 0.6 Ma and drained to the ocean south of Site 1018. At 0.6 Ma, the lake cut an outlet through what is now San Francisco Bay, opening up the modern Sacramento River drainage. Thus, the record at Site 1018 may reflect not only climate and tectonics but also the geologic history of terrestrial sedimentary basins in California.