High-resolution stratigraphy in late Pleistocene/Holocene sediments of the Vema Channel

Abstract

Twenty-five giant gravity cores (GGC) were obtained from the southwest flank of the Rio Grande Rise in the summer of 1980 and were analyzed for time-slice interpretations of the late Pleistocene paleoceanography of the southwestern Atlantic. The cores span a depth interval of 1430–4148 m, encompassing each of the principal deep-water masses. The suite of cores contains a continuous depositional record of the past 150,000–200,000 years. From calcium carbonate, oxygen isotope, carbon-14, and biostratigraphic analyses, it is possible to determine the precision in core-to-core correlation which is obtainable within this region of relatively low accumulation rates (0.5-2.0 cm 10 −3 yrs). Calcium carbonate analyses were performed on each of the 25 cores at 1 cm sampling intervals, using a technique that yields an analytical precision of ±0.25%. The carbonate curves show glacial/interglacial amplitudes of typically 10–30% CaCO 3. Higher-frequency amplitudes of 2–5% are also present. Both the glacial/interglacial and the higher-frequency carbonate signals can be correlated from core to core over the entire depth interval of the transect and are essentially synchronous based on independent stratigraphic control provided in selected cores by δ 18O, 14C, and biostratigraphic analyses. Despite the low sedimentation rates within this particular region, we obtain a precision in core-to-core correlation of ±1000–2000 yrs over the late Pleistocene/Holocene (<30,000 yrs B.P.) and ±4000-10,000 yrs over the late Pleistocene from 30,000 to 150,000 yrs B.P. Comparison of the carbonate stratigraphy with the planktonic foraminiferal δ 18O record shows that there is an apparent lag of approximately 4000 yrs between the δ 18O and CaCO 3 curves at Terminations I and II but no lag at full glacial or interglacial times. These high-resolution studies from the Vema Channel have important implications for interpreting the effects of bioturbation on a given stratigraphic sequence. Modeling the effects of bioturbation on the abundance and isotopic records of the benthonic foraminifera Planulina wuellerstorfi suggests that, for the region of the Vema Channel/Rio Grande Rise, invalid stratigraphic and paleoceanographic interpretations may result if abundance variations in this species are ignored. The results of these analyses show that it is possible to generate detailed stratigraphies in regions that have not previously been selected for high-resolution studies solely because of relatively low accumulation rates of sediments. Our results indicate that carbonate analyses in some regions may provide a distinct advantage over δ 18O analyses because of the ease of analysis and high precision for identifying low-amplitude, high-frequency fluctuations which can be correlated from core to core. At the same time, independent stratigraphic control (via δ 18O or 14C) is essential to verify whether this technique of carbonate curve-matching is valid.