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Oceanic Anoxic Events

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):

  1. The response of the oceanic reservoir to increased sedimentary burial of organic carbon (as determined by the inferred increase in seawater 13C) lags significantly behind black shale deposition during the Selli Event.
  2. The onset of black shale deposition is associated with an extreme, short-lived negative 13C excursion and carbonate dissolution spike that have been attributed to rapid greenhouse gas release (possibly methane as is hypothesized for the LPTM).
  3. The foregoing events are associated with the onset of a pronounced decline in global seawater 87Sr/86Sr to its least radiogenic value in the past 125 m.y. (a second post-Jurassic minimum occurs around the time of OAE-2), suggesting a link between OAEs and oceanic plateau emplacement (Jones et al., 1994; Sinton and Duncan, 1997; Kerr, 1998) (Fig. F8).

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:

  1. What is the history of OAEs in the tropical Atlantic as recorded on Demerara Rise? For example, was OAE-3 restricted to the upper Coniacian or composed of multiple subevents analogous to OAE-1?
  2. What was the duration and vertical extent of specific OAEs in the tropical Atlantic? Results from Demerara Rise could be used in conjunction with high-resolution records for OAE-2 from the shallow-water Tarfaya Basin (Kuhnt et al., 2001) to test (a) the predictions of the oxygen minimum zone model and (b) what role, if any, is played by equatorial divergence in OAE-forcing.
  3. How does the type of Corg differ (a) between different OAE intervals and (b) between these and non-OAE intervals? The proposed transect would provide an opportunity to examine the constraints that can be applied using modern geomicrobiological techniques.
  4. What were the proximal and underlying causes of Cretaceous OAEs? In particular, it is important to determine whether carbon-cycle perturbations are the instigators or merely the consequence of OAEs. The location of Demerara Rise would also allow the competing roles of gateway opening, plateau emplacement, and the hydrological cycle to be evaluated.
  5. Are hypothesized increases in productivity during Cretaceous OAEs real? Current models of OAEs rely heavily on bulk carbonate 13C records from epicontinental sea-land sections where preservation of microfossils is generally poor. Well-preserved microfossils from Demerara Rise would provide a way to test these records and their conventional interpretations by allowing the production of new types of data sets (e.g., 13C).
  6. What mechanism(s) are responsible for the lead and lag effects observed in existing records between the onset of Corg burial and the geochemical response (increase in seawater 13C)? The Demerara Rise transect could address the following two competing hypotheses: (a) globally significant Corg burial began earlier elsewhere (e.g., core of the tropics) than has hitherto been appreciated and (b) some unknown factor acted to buffer seawater 13C values during Corg burial.
  7. What evidence (e.g., negative 13C excursions and depth-transect records of changes in the carbonate compensation depth [CCD]) exists to support the hypothesis that OAEs were driven by the sudden release of greenhouse gases (e.g., CH4, as hypothesized for the LPTM)?
  8. Currently, a wide range of hypotheses invoke changes in ocean circulation and/or stratification to explain OAEs, but virtually no reliable geochemical data exist to constrain changes in the basic physical properties (temperature and salinity) of the water masses involved. These competing hypotheses could be tested via 18O and trace element records using well-preserved microfossils from Demerara Rise.
Biotic Turnover

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 (Eocene–Oligocene 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:

  1. Are leads and lags discernible (on the scale of ~10 k.y. or more) in the pattern of turnover between different groups of plankton and benthos that could elucidate the nature of gradual shifts in climate around a turnover pulse?
  2. Are some species present only during transient climate shifts, and if so, how does their ecology (judged from faunal and isotopic data) compare with closely related species before and after the climatic anomaly? Answers could address (a) the rate of evolutionary response to climatic transients, (b) the magnitude or type of events needed to prompt evolutionary response, and (c) the extent to which species can accommodate environmental change by shifts in ecology rather than evolution (or extinction).
  3. Are biotic changes permanent, or are major evolutionary changes offset from the transient climate shift? For example, the K/T extinction was abrupt, but the subsequent pattern of rediversification occurred over several million years. The long recovery appears to reflect structural changes in ecosystems wrought by the mass extinction (e.g., D'Hondt et al., 1998).
  4. Are particular taxonomic groups more susceptible to extinction or radiation during turnovers?
  5. Do different events (such as the various OAEs) generate predictable patterns of turnover within and between taxonomic groups? For example, thermocline dwelling species and those with complex life histories are believed to be particularly susceptible to extinction (and subsequent radiation) during OAEs (e.g., Hart, 1980; Caron and Homewood, 1983; Leckie, 1987), but these hypotheses have not been tested in detail with stable isotopic data.

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