Elevated primary productivity of calcareous nannoplankton associated with ocean anoxic event 1b during the Aptian/Albian transition (Early Cretaceous)

Abstract

[1] Previous isotopic investigations of Aptian/Albian oceanic anoxic event (OAE) 1b from the western North Atlantic (Blake Nose) posited that increased sea surface temperatures and decreased salinity led to stratification of the upper water column, resulting in lowered dissolved oxygen and enhanced organic matter preservation. We examined calcareous nannofossils from the same site in the western North Atlantic (Blake Nose) to evaluate changes in surface water conditions prior to, during, and after the Aptian/Albian OAE1b. The results of our analysis conflict somewhat with the previous interpretation that OAE1b at Blake Nose was linked to water column stratification. The signal from calcareous nannofossils indicates that in the late Aptian, prior to OAE1b, oligotrophic conditions in the surface waters were replaced by mesotrophic conditions that persisted throughout the OAE event and into the early Albian. We speculate that increased surface water productivity facilitated to some degree the development of OAE1b at Blake Nose.

Planktic Foraminiferal Species Turnover Across Deep-Sea Aptian/ Albian Boundary Sections

“…Resolution of this discrepancy will require further effort to locate better-preserved foraminifera from the uppermost Aptian and basal Albian of Site 763, as test recrystallization and infilling have largely obliterated the Aptian wall microstructures of the specimens studied so far. The global reduction in the relative abundance of planktic foraminifera followed by abrupt extinction of Aptian species (including a dramatic decrease in shell size, maximum shell thickness, and shell ornamentation) is accompanied by a decrease in abundance of robust, oligotrophic environmental indicator species of calcareous nannofossils and replacement by small-sized, high fertility indicator species (Browning and Watkins, 2008). This suggests a common cause, which may have been related to a change in the carbonate chemistry of the mixed layer, making shell calcification more difficult.…”

“…Wavy horizontal lines represent position of unconformities. Calcareous nannofossil biozonation (CC zones) from Browning and Watkins (2008) for Site 1049 and Bralower et al (1997) for Sites 390 and 392A. Ca 5 Campanian.…”

“…columnata of Bown, in Kennedy and others, 2000), Browning and Watkins (2008) located the Aptian/Albian boundary in Hole 1049C at the base of the OAE 1b black shale 2.09 m above the white chalk/gray claystone contact that marks the Pa. eubejaouaensis extinction. While essentially no change in calcareous nannofossil assemblages was identified at the base of the black shale, a marked change in relative abundance and coccolith size was recorded by these authors at the white chalk/claystone contact.…”

Black shale deposition and early diagenetic dolomite cementation during Oceanic Anoxic Event 1: The mid-Cretaceous Maracaibo Platform, northwestern South America

“…Interestingly, V/(VϩNi) correlates with other parameters of our PCA that are interpreted here as soil-derived terrigenous sources, and thus probably delivered to the depositional setting by monsoonal episodes. Such climatic perturbations were seemingly common in mid-Cretaceous tropical zones (Lehmann and others, 1999;Herrle and others, 2003;Browning and Watkins, 2008;Föllmi, 2012;Wagner and others, 2013), and would have delivered soil-sourced vanadium to the shallow anoxic ramp at the time of transgressive black shales alteration to marlstones. Due to the general redox conditions governing early diagenesis of the ramp, vanadium was not largely released from the organic-rich sediments.…”

“…Terrigenous influx and changes in salinity.-As observed in other tropical and subtropical mid-Cretaceous sequences (Lehmann, and others, 1999;Herrle and others, 2004;Browning and Watkins, 2008), monsoonal forcing mechanisms would have led to a transient influx of freshwater and finer-grained detritus to the inner ramp. This caused the development of a schizohaline environment (Folk and Siedlecka, 1974).…”