Late Albian paleoceanography of the western subtropical North Atlantic

Abstract

[1] A late Albian-early Cenomanian record ($103.3 to 99.0 Ma), including organic-rich deposits and a d 13 C increase associated with oceanic anoxic event 1d (OAE 1d), is described from Ocean Drilling Program sites 1050 and 1052 in the subtropical Atlantic. Foraminifera are well preserved at these sites. Paleotemperatures estimated from benthic d 18 O values average $14°C for middle bathyal Site 1050 and $17°C for upper bathyal Site 1052, whereas surface temperatures are estimated to have ranged from 26°C to 31°C at both sites. Among planktonic foraminifera, there is a steady balance of speciation and extinction with no discrete time of major faunal turnover. OAE 1d is recognized on the basis of a 1.2% d 13 C increase ($100.0-99.6 Ma), which is similar in age and magnitude to d 13 C excursions documented in the North Atlantic and western Tethys. Organic-rich ''black shales'' are present throughout the studied interval at both sites. However, deposition of individual black shale beds was not synchronous between sites, and most of the black shale was deposited before the OAE 1d d 13 C increase. A similar pattern is observed at the other sites where OAE 1d has been recognized indicating that the site(s) of excess organic carbon burial that could have caused the d 13 C increase has (have) yet to be found. Our findings add weight to the view that OAEs should be chemostratigraphically (d 13 C) rather than lithostratigraphically defined.

Depth-habitat reorganization of planktonic foraminifera across the Albian/Cenomanian boundary

“…The lithology of Hole 1050C changes from claystone below to chalk above the A/C boundary. A coring gap at 556.6-558.0 mbsf (Core 25/26), in which an unconformity is present, translates to a sedimentation gap of ,0.8 Myr based on the graphic correlation with shallower Site 1052 (Petrizzo and Huber 2006;Petrizzo et al 2008). It explains the limited occurrence of the OAE 1d black shales at this site as compared to Site 1052 (Wilson and Norris 2001;Petrizzo et al 2008).…”

“…Systematic A-C paleoceanographic studies carried out at North Atlantic ODP Sites 1050 and 1052 of the Blake Nose depth transect ( Fig. 1) (Wilson and Norris 2001;Watkins et al 2005;Petrizzo et al 2008) and Deep Sea Drilling Project (DSDP) Site 547 off Morocco (Nederbragt et al 2001) have revealed that stratigraphic distribution of planktonic foraminifera in both regions and of nannofossils in the eastern North Atlantic are consistent with the global compilation by Leckie et al (2002). In the western North Atlantic, on the other hand, nannoplankton at the time of OAE 1d suffered local extinctions of 20% of taxa (biased to deep-dwelling representatives), preceded by a long-term increase in high-fertility indicative taxa, suggesting regional upwelling and/or deep mixing (Watkins et al 2005).…”

“…Traditionally, the strategy for depthecology ranking of planktonic taxa was twofold: (1) comparison with modern morphological counterparts (small, simple, globular, Globigerina-like groups are attributed to the shallow-dwelling assemblage, whereas large, ornamented, flattened, Globorotalia-like groups are attributed to the deep-dwelling assemblage [Hart 1980[Hart , 1999Caron and Homewood 1983;Premoli Silva and Sliter 1999]); and (2) biogeographic distributions (taxa common in epicontinental shelf settings are attributed to the shallow-dwelling assemblage, whereas taxa that dominated in pelagic settings are attributed to the deep-dwelling assemblage [Hart and Bailey 1979;Leckie 1987]). Although these approaches have long been considered appropriate and were generally supported by available stable isotope data, a growing number of exceptions have been documented by recent measurements of glassy foraminifera (Norris and Wilson 1998;Wilson and Norris 2001;Bornemann and Norris 2007;Petrizzo et al 2008). For example, the Albian genus Planomalina, which by its single-keeled morphology would be identified as a thermocline dweller, exhibits isotopic features of an upper-ocean habitat.…”

“…Subsequent data generated from clayhosted foraminifera exhibiting exceptional ''glassy'' preservation (sensu Sexton et al 2006) have yielded significant insights into mid-Cretaceous paleoceanography. The major achievements include reconciliation of the so-called cool tropics paradox (Norris and Wilson 1998;Wilson and Norris 2001), establishment of the Turonian hyperthermal period (Huber et al 2002;Norris et al 2002;Wilson et al 2002;Bice et al 2003; see also Bornemann et al 2008), and recognition of short-term intensification (Erbacher et al 2001) or destabilization (Wilson and Norris 2001;Petrizzo et al 2008) of vertical stratification.…”

“…O records of foraminifera at Site 1052 (Wilson and Norris 2001;Petrizzo et al 2008). Still, the proposed forcing mechanisms of OAE 1d are site-specific, resulting from increased primary productivity with disrupted stratification as proposed for the western North Atlantic (Watkins et al 2005), and/or elevated runoff leading to enhanced stratification and poor bottom-water oxygenation, such as in the Mediterranean Tethys (Vocontian Basin, France) (Bornemann et al 2005).…”