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General Early Cretaceous Paleoceanography
Microplankton Evolution

The Early and mid-Cretaceous were critical times in the evolution of planktonic foraminifers and calcareous nannoplankton (e.g., Roth, 1987; Leckie, 1989; Premoli Silva and Sliter, 1999). Nannoplankton underwent dramatic radiations close to the Jurassic/Cretaceous and Barremian/Aptian boundaries (e.g., Bralower et al., 1989, 1994). Both of these events have been documented in the Atlantic and Tethys, but not yet from the Pacific. Pacific sites recording these diversification events would help provide an understanding of their causes.

Planktonic foraminifers appear to have evolved in the Bajocian (Middle Jurassic), but their occurrence is sporadic below the Lower Cretaceous. The diversification of this group was, until recently, thought to have occurred in the early Aptian. Coccioni and Premoli Silva (1994), however, found the evolutionary appearance of a number of taxa far below their previous ranges in the lower Valanginian of the Rio Argos section of Spain. Documentation of this diversification event in other locations and oceanographic settings will help our understanding of its causes.

Shatsky Rise drilling will help us answer the following questions: (1) How did the evolution of nannoplankton correlate to changes in ocean thermal structure and circulation? and (2) Is there evidence for diversification of planktonic foraminifers in the early Valanginian as in Spain, and if so, did this event correlate with any obvious changes in circulation or climate?

Valanginian Greenhouse Event

A major change in stable carbon isotope ratios of marine carbonates and organic matter has been observed in the Valanginian (e.g., Lini et al., 1992). The event appears to correlate with a major burial event of Corg, an increase in atmospheric CO2, and global warming, perhaps the earliest indications of the Cretaceous "greenhouse" climate (Lini et al., 1992). Increased crustal production rates at this time (e.g., Larson, 1991b) suggest that the event may have a volcanic origin. Warming in the Valanginian is at odds with the evidence of Stoll and Schrag (1996) and others for glaciation in this part of the Cretaceous. Recovery of high-quality stratigraphic sections from additional locations will help resolve this issue. Shatsky drilling will help us address how the Valanginian carbon isotope record correlates to indicators of climate change and volcanism and whether there is evidence for warming or cooling in this time interval.

Early Cretaceous CCD Fluctuations

The Early and mid-Cretaceous were characterized by major changes in the level of the CCD (e.g., Thierstein, 1979; Arthur and Dean, 1986). These changes likely resulted from changes in fertility, sea level, ocean floor hypsometry, and ocean circulational patterns. One of the most dramatic events occurred in the early Aptian, at around the same time as the massive Pacific volcanic event, suggesting that volcanism played a direct role, perhaps through increased pCO2. The few data that exist for the Pacific suggest a different CCD history from the Atlantic (Thierstein, 1979), and more data will help resolve the history of the Pacific CCD. Shatsky Rise drilling will help us address the following questions: (1) What was the gradient of carbonate dissolution in the mid-Cretaceous Pacific Ocean? (2) What was the history of variation in the lysocline and CCD in the Early and mid-Cretaceous? and (3) Was a major early Aptian CCD shoaling episode observed for the Atlantic Ocean basins characteristic of the global ocean, or were the oceans out of phase as the result of the pattern of deep-water aging?

Nature and Age of Shatsky Rise Basement

LIPs such as the Ontong Java Plateau and Shatsky Rise were constructed during voluminous magmatic events that took place over geologically brief (<1 m.y.) time intervals (e.g., Duncan and Richards, 1991; Tarduno et al., 1991; Coffin and Eldholm, 1994). These events are thought to be associated with massive thermal anomalies in the mantle known as "superplumes" (Larson, 1991b). A likely possibility is that the voluminous phase of superplume activity was associated with the ascent of a plume "head" and that activity declined as the magma source dried up as the lithosphere rode over the plume "tail." One of the major questions concerning the origin of LIPs such as Shatsky Rise is whether they formed in a midplate setting or at a divergent boundary, possibly a triple junction, at times of changing plate geometry (e.g., Sager et al., 1988).

Trace element geochemistry of most samples from Shatsky Rise are close to mid-ocean-ridge basalt (MORB) (Tatsumi et al., 1998), indicating that they were generated at a divergent boundary, but a few samples have an affinity closer to Polynesian alkalic basalts, suggesting a midplate origin. The latter result is not unexpected, since Shatsky Rise is thought to have formed in the South Pacific (McNutt and Fischer, 1987) near crust with the distinctive Polynesian chemistry. Additional basement drilling at Shatsky Rise will help us address whether it had some kind of a hybrid origin or whether there are other explanations for the few anomalous samples.

Although volcanic basement crops out at several localities on Shatsky Rise (Sliter et al., 1990), basement samples obtained are from dredges, and one pebble in a core-catcher sample from DSDP Site 50, and all of these are heavily weathered. Ozima et al. (1970) dated volcanic rocks dredged from Shatsky Rise as Tertiary in age. Either these rocks were derived from late stage volcanism identified in seismic data from the rise, or possibly their pervasive alteration precludes reliable age determination. Maximum estimates for the age of basement on Shatsky Rise can be obtained by adjacent magnetic anomalies (Nakanishi et al., 1989). These ages range from 148 Ma (Late Jurassic polarity Zone CM21) at the Southern High to 136 Ma (Berriasian–Valanginian polarity Zone CM14) at the Northern High based on the timescale of Gradstein et al. (1994). Fresh basement samples will provide valuable age information.

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