Spatial linkages between coral proxies of terrestrial runoff across a large embayment in Madagascar

Abstract

Coral cores provide vital climate reconstructions for site-specific temporal variability in river flow and sediment load. Yet, their ability to record spatial differences across multiple catchments is relatively unknown. Here, we investigate spatial linkages between four coral proxies of terrestrial runoff and their relationships between sites. Coral cores were drilled in and around Antongil Bay, the largest bay in Madagascar, and individually analysed for fifteen years of continuous luminescence (G / B), Ba / Ca, δ¹⁸O_sw and δ¹³C data. Each coral core was drilled close to individual river mouths (≥ 7 km), and proxy data were compared to modelled river discharge and sediment runoff data for the three corresponding catchments. A reasonable agreement between terrestrial runoff proxies with modelled river discharge and sediment yield was observed. Some inconsistencies between proxy and modelled data are likely linked to proxy behaviour, watershed size and local environmental physiochemical parameters. In general, the further a coral resided from its river source, the weaker the proxy relationship was with modelled data and other corals, due to mixing gradients and currents. Nevertheless, we demonstrate that two coral Ba / Ca and luminescence (G / B) records influenced by the same watershed are reproducible. Furthermore, a strong Ba / Ca relationship was observed between two cores from distant watersheds, with baseline averages in agreement with modelled sediment runoff data. As humic acids behave conservatively in the water column, luminescence (G / B) data gave the highest regional correlations between cores, and showed the most consistent relationship with site specific modelled discharge. No statistical relationship was observed between cores in terms of interannual δ¹⁸O_sw and δ¹³C, meaning corals were recording a localised signal at their respective sites, confounded by vital effects. Comparing proxy baseline averages and mean seasonal cycles provided a good overview of the runoff dynamics of the bay system.