OAEs represent major disruptions to the ocean system defined by massive deposition of organic carbon in marine environments (Schlanger and Jenkyns, 1976; Jenkyns, 1980; Arthur et al., 1990). Despite the fundamental role that OAEs are widely hypothesized to have played in the evolution of Earth's climatic and biotic history, very little is really known about the causes and effects of these events. Arguably, between two and six OAEs occurred during the mid- to Late Cretaceous (OAE-1a through OAE-1d, OAE-2, and OAE-3) (Jenkyns, 1980; Arthur et al., 1990, Erbacher et al., 1997) (Fig. F8), and these are particularly important because they have left records not merely in shallow seas but also in the deep oceans.
Records of 13C from the Western Interior, the English Chalk, and Italian Scaglia appear to confirm the initial designation of OAE-3 for the late Coniacian, but current resolution of Atlantic records is insufficient to determine the existence of additional events in the late Turonian through Santonian (Jenkyns, 1980; Jenkyns et al., 1994). Similarly, until recently, comparatively little was known about the Albian OAEs (OAE-1b through OAE-1d), but two new studies demonstrate the potential to improve constraints on the origin of different OAEs when diagenetically uncompromised microfossils become available from modern ocean drilling. Data from ODP Site 1049 suggest that pronounced water column stratification instigated OAE-1b (Erbacher et al., 2001), whereas records from nearby Site 1052 indicate that OAE-1d was triggered by the total collapse of upper ocean stratification, intense vertical mixing, and high oceanic productivity (Wilson and Norris, 2001). These antipodal hypotheses for the proximal causes of two OAEs within the same stage emphasize the utility of targeting sections that we know to contain records of multiple OAEs.
The two most prominent mid- to Upper Cretaceous black shale events are the late early Aptian Selli Event (OAE-1a; ~120 Ma) and the Cenomanian/Turonian boundary, Bonarelli Event (OAE-2; ~93.5 Ma) (Fig. F6). Both OAE-1a and OAE-2 have sedimentary records in all ocean basins (Arthur et al., 1985, 1988, 1990; Bralower et al., 1994; Thurow et al., 1992), and the Aptian event is now known to have been truly cosmopolitan; its sedimentary expression extended even to the extremely shallow waters of mid-Pacific atolls (Jenkyns and Wilson, 1999). These findings and recent improvements to 13C records from classic European sections (both in outcrop and drill core) and the mid-Cretaceous seawater 87Sr/86Sr curve reveal three important factors concerning the possible origins of OAEs (Bralower et al., 1997; Menegatti et al., 1998; Erba et al., 1999):
The global occurrence of laminated sediments and a variety of geochemical records demonstrate that the response of the carbon cycle during OAE-2 was somehow related to dysoxic to euxinic conditions at the sediment/water interface (e.g., Sinninghe et al., 1998). However, the cause and dimensions of O2-deficiency remain unclear and controversial. The substantial positive 13C excursion of seawater at the time of OAE-2 (Scholle and Arthur, 1980; Schlanger et al., 1987; Jenkyns et al., 1994) has also been attributed to increased global oceanic productivity and increased rates of Corg burial. This process of sedimentary sequestration of Corg is hypothesized to act as a rapid negative feedback mechanism for global warming via drawdown of atmospheric carbon dioxide (Arthur et al., 1988; Kuypers et al., 1990).
The condensed section at DSDP 144 contains black carbonaceous claystones and shales correlative to at least three Cretaceous OAEs. Records of at least five OAEs (OAE-1b, -1c, -1d, -2, and -3) probably can be penetrated by transect drilling on the Demerara Rise. The following scientific questions will be addressed by drilling this transect:
The tropics are widely viewed as an environment in which physiochemical factors and thus biotic compositions are inherently stable. Yet many low-latitude species have low environmental tolerances, thereby suggesting that relatively small climate changes may result in a substantial biological response (Stanley 1984). The so-called Cretaceous and Paleogene greenhouse was characterized by a series of significant marine and terrestrial biotic turnovers. Most of these events seem to be linked to major changes in Earth's climate (EoceneOligocene transition and LPTM), paleoceanography, and/or the geochemical carbon cycle (Cretaceous OAEs and mid-Maastrichtian Event). Many of these events also produced synchronous turnovers in both terrestrial and marine biotas. The causes of most of these turnovers are poorly known because of the absence of expanded sections in the deep sea, where paleontological and isotopic studies can be carried out at high temporal resolution.
The biotic turnovers of the mid-Cretaceous OAEs (OAE-1b, -1d, and -2) are broadly comparable to one another even if the detailed causal factors are thought to have been different (Leckie, 1987; Erbacher and Thurow, 1997; Premoli Silva et al., 1999). A faunal crisis in nannoconids is well documented in the Aptian (Erba, 1994). Similarly, the early Albian OAE-1b strongly influenced the evolution of both planktonic foraminifers and radiolarians, as did the other OAEs. Some events not only influenced planktonic groups but also benthic foraminifers, ammonites, bivalves, and even angiosperms, and OAE-2 ranks as one of the eighth largest mass extinctions in Phanerozoic Earth history (Sepkoski, 1986). Extension of the oxygen minimum zone and a rapid eutrophication of the oceans has been linked to extinction and a subsequent radiation of plankton and benthos alike (e.g., Hart, 1980; Caron and Homewood, 1983; Kaiho et al., 1994; Erbacher et al., 1996; Leckie, 1989). 13C excursions around three events (OAE-1b, -1d, and -2) are interpreted in terms of increases in oceanic productivity, and this mechanism has been invoked to explain wide-scale carbonate platform drowning events in the Tethyan realm (Erbacher and Thurow, 1997; Weissert et al., 1998). In contrast, results from the Pacific suggest that high tropical sea-surface temperatures rather than eutrofication were responsible for platform drowning (Wilson et al., 1998; Jenkyns and Wilson, 1999). Cretaceous OAEs and extreme climates of the Paleogene (Cretaceous/Tertiary [K/T] boundary, LPTM, and middle to late Eocene refrigeration) led to profound changes in plankton and benthos in the oceans (Thomas, 1998; Aubry, 1998). The following questions will be addressed using well-preserved Demerara Rise microfauna:
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