LEADS, LAGS, AND CORRELATIONS AMONG GEOCHEMICAL RECORDS THROUGH THE CENOMANIAN/TURONIAN BOUNDARY INTERVAL IN DEMERARA RISE BLACK SHALES

Abstract:

Ocean anoxic event 2 (OAE2) represents the largest perturbation of the carbon cycle of the past 250 million years. At many localities OAE2 is marked by large geochemical excursions and coincides with a change from bioturbated, organic-poor deposits to laminated, organic-rich deposits. The adage that correlation does not demonstrate causation is often and appropriately invoked in discussing geologic records such as these. On the other hand, cause and effect relationships do lead to correlation, and such observations are robust, objective, and potentially insightful.

Samples from Demerara Rise provide an excellent opportunity to examine details of correlation among OAE2 geochemical records. The Demerara Rise record is largely complete across 10 million years, and deposits are comprised of > 100 m of finely laminated, carbonaceous silty claystones at each of 5 sites that together represent a 1 km thick paleodepth transect. Preservation of microfossils and organic matter is excellent, and a number of geochemical trends have been well characterized at these sites. We will focus on the carbon, nitrogen, oxygen, neodymium, molybdenum, and osmium records from two sites. Oxygen isotopes suggest the water column had a relatively cool mid-water zone and warmer surface and bottom waters throughout the interval studied. Demerara Rise samples also consistently exhibit low to very low δ¹⁵N_total values (-1 to -3.5‰), but during OAE2 only very low δ¹⁵N_total values (< -2.5‰) are present. The initiation of the δ¹⁵N_total shift coincides with the initiation of the OAE2 δ¹³C_org excursion, but the δ¹⁵N_total shift lasts longer. Os isotopic ratios and concentrations both change dramatically at the initiation of the δ¹³C_org excursion and correlate closely with similarly dramatic changes in Mo and Nd records at the deeper site. At the shallower site, though, the Nd change lags the Os and δ¹³C_org shifts, and the Mo shifts during the OAE2 interval are modest and of similar magnitude to variations outside the OAE2 interval. Integrating these data sets seems to defy a simple, single-cause explanation. Instead they suggest explaining the OAE2 record may require a complicated but resolvable sequence of events that includes conducive starting conditions, novel inputs, and circulation changes.