Niels A.G.M. van Helmond

Spatial extent and degree of oxygen depletion in the deep proto-North Atlantic basin during Oceanic Anoxic Event 2

Massive organic matter burial due to widespread ocean anoxia across the Cenomanian/Turonian boundary event (∼94 Ma) resulted in a major perturbation of the global carbon cycle: the so-called Oceanic Anoxic Event 2 (OAE2). The characteristics and spatial distribution of the OAE2 deposits that formed in the deep basin of the proto-North Atlantic remain poorly described, however. Here we present proxy data of redox sensitive (trace) elements (e.g., Mo, Fe/Al, Corg/Ptot, and Mn) for OAE2 sediments from five Deep Sea Drilling Project and Ocean Drilling Program sites located in the deep proto-North Atlantic basin. Our results highlight that bottom waters in the entire deep proto-North Atlantic were anoxic during most of OAE2. Furthermore, regressions of Mo with total organic carbon content (TOC), previously shown to document the degree of water mass restriction, confirm that the water circulation in the proto-North Atlantic basin was severely restricted during OAE2. Comparison of these values to Mo/TOC ratios in the present-day Black Sea suggests a renewal frequency of the deep proto-North Atlantic water mass of between 0.5 and 4 ka, compared to a maximum of ∼200 years for the present-day northern Atlantic. The Plenus Cold Event, a cooler episode during the early stages of OAE2 hypothesized to be caused by declining pCO2 due to extensive burial of organic matter, appears to have led to temporary re-oxygenation of the bottom water in the deep proto-North Atlantic basin during OAE2.

Shelf hypoxia in response to global warming after the Cretaceous-Paleogene boundary impact

The Chicxulub asteroid impact at the Cretaceous-Paleogene (K-Pg) boundary resulted in one of the most abrupt global warming events in the past 100 m.y., presenting an analogue to current global warming. Here, we present high-resolution geochemical, micropaleontological, and palynological records of the Brazos-1 (Texas, USA), Stevns Klint (Denmark), and Caravaca (Spain) K-Pg boundary sections to assess the rapid environmental changes during the global warming following the brief K-Pg boundary impact winter. Warming during the first millennia after the impact is associated with hypoxic bottom waters at the studied shelf sites, as indicated by molybdenum enrichments, causing major stress for benthic communities. We attribute this decline in dissolved oxygen to a combination of decreased gas solubility and ocean ventilation resulting from the warming of the sea water, and increased oxygen demand in shelf bottom waters due to increased nutrient inputs and associated high productivity.