Increased thermohaline stratification as a possible cause for an ocean anoxic event in the Cretaceous period

Abstract

Ocean anoxic events were periods of high carbon burial that led to drawdown of atmospheric carbon dioxide, lowering of bottom-water oxygen concentrations and, in many cases, significant biological extinction. Most ocean anoxic events are thought to be caused by high productivity and export of carbon from surface waters which is then preserved in organic-rich sediments, known as black shales. But the factors that triggered some of these events remain uncertain. Here we present stable isotope data from a mid-Cretaceous ocean anoxic event that occurred 112 Myr ago, and that point to increased thermohaline stratification as the probable cause. Ocean anoxic event 1b is associated with an increase in surface-water temperatures and runoff that led to decreased bottom-water formation and elevated carbon burial in the restricted basins of the western Tethys and North Atlantic. This event is in many ways similar to that which led to the more recent Plio-Pleistocene Mediterranean sapropels, but the greater geographical extent and longer duration (approximately 46 kyr) of ocean anoxic event 1b suggest that processes leading to such ocean anoxic events in the North Atlantic and western Tethys were able to act over a much larger region, and sequester far more carbon, than any of the Quaternary sapropels.

Controls on phosphorus regeneration and burial during formation of eastern Mediterranean sapropels

“…Thomson et al, 1999;Nijenhuis et al, 1999;Erbacher et al, 2001), these results may also have wider implications. In their model calculations for global ocean C, P and O cycling, Ingall (1994, 1996) assumed that CFA formation and burial increased substantially during ocean anoxia as a result of increased primary production.…”

Confounding effects of oxygen and temperature on the TEX 86 signature of marine Thaumarchaeota

“…Although some increase in temperature during OAEs and the PETM is consistent with paleoclimate model simulations and planktonic foraminiferal δ 18 O-based SST reconstructions, TEX 86 values (as high as 0.95) and TEX 86 -derived temperatures (up to 43°C) observed during these low-oxygen events seem anomalously high (31,(53)(54)(55)(56)(57)(58). However, because δ 18 O-based temperature estimates are not available throughout the black shales of an OAE, there is no independent validation of the exceptionally warm TEX 86 -inferred temperatures for these climate periods (31,59,60).…”

Evidence for photic‐zone euxinia in the Early Albian Santana Formation (Araripe Basin, NE Brazil)

“…In such a model, enhanced productivity is triggered by high riverine nutrient fluxes and, in combination with salinity-driven surface water stratification and limited vertical mixing, results in oxygen-deficient conditions within the water column and at the sea floor. The establishment of such a thermohaline stratification has been proposed to explain enhanced organic-carbon accumulation in Mediterranean sapropels and the Cretaceous OAE 1b in the western North Atlantic (Erbacher et al, 2001;Meyers, 2006).…”

“…Alternatively, or additionally, the deposition of organic-rich sediments and the establishment of photic-zone euxinia in the Araripe Basin may have been linked to a protracted phase of oceanic anoxia in the nearby Central and South Atlantic. The Early Albian was characterized by a widespread and long-lasting episode of black shale formation (OAE 1b and associated events) documented in pelagic and hemipelagic environments from the Tethys Ocean as well as from the evolving Atlantic (Erbacher et al, 2001;Leckie et al, 2002). A possible scenario would thus include the intrusion of an anoxic (or at least oxygen deficient) water mass from the adjacent evolving Atlantic Ocean into the north-eastern Brazilian rift system during a major transgressive event.…”