Stable oxygen and carbon isotope data for planktic and benthic foraminifera from southern Atlantic of DSDP Hole 74-525A and ODP Hole 113-690C

Published stable isotope records in marine carbonate are characterized by a positive d18O excursion associated with a negative d13C shift during the early Maastrichtian. However, the cause and even the precise timing of these excursions remain uncertain. We have generated high-resolution foraminiferal stable isotope and gray-scale records for the latest Campanian to early Maastrichtian (73-68 Ma) at two Ocean Drilling Program sites, 525 (Walvis Ridge) and 690 (Weddell Sea). We demonstrate that the negative d13C excursion is decoupled from the d18O increase with a lag of about 600 ka. Our d13C records (both planktic and benthic) show an amplitude for the negative excursion of 0.7 per mill that falls between about 72.1 and 70.7 Ma. Our planktic d18O records indicate an overall increase of 1.2 per mill from 73 to 68 Ma at Site 690, whereas at Site 525 they record a slightly smaller increase (1 per mill) that peaks around 70.1 Ma with decreasing values thereafter. Our benthic d18O data indicate an increase of 1.5 per mill at Site 525 and 0.7 per mill at Site 690 between about 71.4 and 69.9 Ma. Benthic d18O values show different baseline values at the two sites before and after the excursion, but the larger increase at Site 525 means that the values attained at the peak of the excursion are similar at the two sites. We interpret this observation in terms of water mass changes. The excursion is interpreted to reflect a cooling of bottom waters in response to the strengthening contribution of intermediate- to deep-water production in the high southern latitudes rather than increased ice volume. The associated carbon cycle perturbations that we observe are interpreted to reflect a weakening of surface water stratification and increased productivity, as supported by our gray value data.