ALBERT

Description: The mechanics of ocean circulation during the Late Cretaceous greenhouse interval remain contested (MacLeod and Hoope, 1992, doi:10.1130/0091-7613(1992)020〈0117:ETIBWB〉2.3.CO;2; Frank and Arthur, 1999, doi:10.1029/1998PA900017; MacLeod and Huber, 2001; Abramovich et al., doi:10.1029/2009PA001843; Isaza-Londono et al., doi:10.1029/2004PA001130; MacLeod et al., 2005, doi:10.1130/G21466.1), with the role of North Atlantic Deep Water in ocean circulation particularly debated: the relative warming of the North Atlantic during the termination of the greenhouse interval has been attributed to heat piracy from North Atlantic Deep Water formation (Isaza-Londono et al., doi:10.1029/2004PA001130; MacLeod et al., 2005, doi:10.1130/G21466.1), but the sources of Cretaceous deep water have been difficult to resolve. Nd isotopes as captured by seafloor sediments and expressed as epsilon-Nd(t) reflect the region in which the water mass was formed. Here we present epsilon-Nd(t) measurements from Cretaceous- to Palaeogene-aged sediments from four cores in the tropical North Atlantic. Before 69 Myr ago, we find extremely low epsilon-Nd(t) values of about -16, consistent with the presence of a warm, saline deep water mass formed in the low latitudes (MacLeod et al., 2008, doi:10.1130/G24999A.1; Jiménez Berrocoso et al., 2010, doi:10.1130/G31195.1). By 62 Myr ago, epsilon-Nd(t) values had risen to -11, similar to values reported from the northern North Atlantic over the past 65 million years, but lower than most contemporaneous values in the South Atlantic (Robinson et al., 2010, doi:10.1130/G31165.1) and Pacific oceans ((MacLeod et al., 2008, doi:10.1130/G24999A.1; Frank et al., 2005, doi:10.1029/2004PA001052 ). We therefore suggest that the epsilon-Nd(t) shift reflects the increasing influence of a northern-sourced water mass at this site, indicating the onset or intensification of deep- or intermediate-water formation in the North Atlantic 69 Myr ago. Our findings support the heat piracy model and imply that circulation patterns during the greenhouse interval were different from those of the subsequent relatively temperate interval.

Description: Neodymium isotopes of fish debris from two sites on Demerara Rise, spanning ~4.5 m.y. of deposition from the early Cenomanian to just before ocean anoxic event 2 (OAE2) (Cenomanian-Turonian transition), suggest a circulation-controlled nutrient trap in intermediate waters of the western tropical North Atlantic that could explain continuous deposition of organic-rich black shales for as many as ~15 m.y. (Cenomanian-early Santonian). Unusually low Nd isotopic data (epsilon-Nd(t) ~-11 to ~-16) on Demerara Rise during the Cenomanian are confirmed, but the shallower site generally exhibits higher and more variable values. A scenario in which southwest-flowing Tethyan and/or North Atlantic waters overrode warm, saline Demerara bottom water explains the isotopic differences between sites and could create a dynamic nutrient trap controlled by circulation patterns in the absence of topographic barriers. Nutrient trapping, in turn, would explain the ~15 m.y. deposition of black shales through positive feedbacks between low oxygen and nutrient-rich bottom waters, efficient phosphate recycling, transport of nutrients to the surface, high productivity, and organic carbon export to the seafloor. This nutrient trap and the correlation seen previously between high Nd and organic carbon isotopic values during OAE2 on Demerara Rise suggest that physical oceanographic changes could be components of OAE2, one of the largest perturbations to the global carbon cycle in the past 150 m.y.