Contrasting variability in foraminiferal and organic paleotemperature proxies in sedimenting particles of the Mozambique Channel (SW Indian Ocean)

Abstract

Accurate sea surface temperature (SST) proxies are important for understanding past ocean and climate systems. Here, we examine material collected from a deep-moored sediment trap in the Mozambique Channel (SW Indian Ocean) to constrain and compare both inorganic (δ 18O, Mg/Ca) and organic ( U 37 k ′ , TEX 86) temperature proxies in a highly dynamic oceanographic setting for application in paleoceanography. High-resolution time-series current velocity data from long-term moorings (2003 – present) deployed across the Mozambique Channel reveal the periodic migration of four to six meso-scale eddies through the channel per year. These meso-scale eddies strongly influence water mass properties including temperature and salinity. Despite the dynamic oceanographic setting, fluxes of the surface-dwelling planktonic foraminifera Globigerinoides ruber and Globigerinoides trilobus follow a seasonal pattern. Temperatures reconstructed from G. ruber and G. trilobus δ 18O and Mg/Ca closely mirror seasonal SST variability and their flux-weighted annual mean SSTs of 28.1 °C and 27.3 °C are in close agreement with annual mean satellite SST (27.6 °C). The sub-surface dwelling foraminifera Neogloboquadrina dutertrei and Globigerinoides scitula recorded high-frequency temperature variations that, on average, reflect conditions at water depths of 50–70 m and 200–250 m, respectively. Concentrations and fluxes of organic compounds (alkenones and crenarchaeol) display no or only moderate seasonality but flux weighted means of the associated temperature signatures, U 37 k ′ , and TEX 86 H of 28.3 °C and 28.1 °C, respectively, also closely reflect mean annual SST. We analyzed all time-series data using multiple statistical approaches including cross-correlation and spectral analysis. Eddy variability was clearly expressed in the statistical analysis of physical oceanographic parameters (current velocity and sub-surface temperature) and revealed a frequency of four to six cycles per year. In contrast, statistical analysis of proxy data from the sediment trap did not reveal a significant coupling between eddy migration and organic compound fluxes or reconstructed temperatures. This is likely a result of the relatively low resolution (21 days) and short (2.5 years) duration of the time series, which is close to the detection limit of the eddy frequency.