Sedimentation and burial of organic and inorganic temperature proxies in the Mozambique Channel, SW Indian Ocean

Abstract

Paleoceanographic studies strongly rely on proxies to reconstruct past environmental conditions. However, several factors influence the reliability of these proxies, particularly during sedimentation and burial. In this study, we measured both inorganic (δ 18O and Mg/Ca on three species of planktonic foraminifera ( Globigerinoides ruber, Globigerinoides trilobus and Neogloboquadrina dutertrei) and organic ( U 37 k ′ and TEX 86) sea surface temperature (SST) proxies on core top sediment taken from an east-west transect across the Mozambique Channel. We contrast our findings with previously published modern time-series temperature proxy data from a sediment trap moored in the mid-channel. The coretop sediment was analyzed for 14C, 210Pb and excess (xs) 234Th. While 234Th xs data indicate a flux of fresh particulate matter to the bottom sediment, radiocarbon dating shows that the core top sediments are composed of material that is on average about 1000 years old. The fine organic carbon is consistently (even though only slightly) younger than the coarser foraminiferal calcite, which is likely caused by preferential downcore mixing of the fine fraction. Besides vertical mixing by bioturbation, stable lead isotope ratios from the time-series particle fluxes indicate episodic lateral transport of old particles from the shelf to the deep Mozambique Channel as an additional source of pre-aged material in core tops. Core top temperature proxies show warmer values close to the channel flanks while colder values are found in the mid-channel. These could be associated with higher maximum summer temperatures in modern coastal waters in contrast to the mid-channel. Additionally, we find an offset in all foraminiferal proxies between core top samples and time-series data that corresponds to 1–3 °C, which probably reflects climate variability over the past ∼2000 years. However, this temperature difference is not observed in the organic proxies U 37 k ′ and TEX 86, which may result from current transport of unconsolidated organic matter that can homogenize the organic proxy signal prior to burial.