OCEANOGRAPHY OF THE WESTERN INTERIOR SEAWAY DURING OAE 2 USING ND ISOTOPES

During the greenhouse climate of the mid-Cretaceous, the Western Interior Seaway (WIS) experienced semi-restricted conditions with poor water-column ventilation, leading to the accumulation of black organic-rich shales. In the Maverick Basin (TX, USA), the southernmost extent of the WIS, the main phase of organic matter deposition occurred in the early to late Cenomanian, before Oceanic Anoxic Event 2 (OAE 2). A sea-level rise prior to the event may have caused the basin to become better ventilated during the Cenomanian–Turonian transition, and ocean circulation likely played a major role on productivity and the preservation of organic matter. Widely different regimes of ocean circulation are suggested to have operated, with alternating incursions of water masses from both the north and the south. Foraminiferal assemblages suggest that during the early phase of OAE 2, Tethyan waters were drawn northward into the WIS, whereas dinocyst occurrences indicate an influx of Boreal surface waters.

Here we present neodymium-isotope records (e_Nd) of fish teeth and detrital fractions from the Eagle Ford Formation that record the presence of distinct water masses at depth and allow testing of suggested mechanisms of ocean circulation. Mid- to late Cenomanian values of e_Nd around -3 are unusually radiogenic compared to coeval open ocean e_Nd records from the North Atlantic and may reflect a strong influence of regional volcanism close to the WIS and/or weathering of mafic igneous rocks in the water-mass source area. An excursion to positive e_Nd values in the WIS during OAE 2 may reflect changes in local weathering, or alternatively, the incursion of water masses carrying a signature of volcanic activity. The coeval emplacement of several Large Igneous Provinces (LIPs), including the High Arctic LIP and the Caribbean LIP, may have influenced the seawater chemistry of the WIS, as reflected in Os and Cr concentrations and isotope ratios from the USGS Portland core. Comparison of seawater and detrital e_Nd signatures with records north and south of the Maverick Basin will help elucidate the direction and degree of deep-water exchange in the southern WIS. Deciphering the role of ocean circulation in organic-matter deposition and preservation will improve our understanding of ocean-climate dynamics influencing the Earth’s carbon cycle.