ALBERT

Description: Reconstructing variations in the intensity of chemical weathering in river basins is crucial if we are to understand how climate change impacts environment and whether there are feedbacks between climate and weathering processes. Quantifying chemical weathering is, however, a complicated process, involving a number of competing proxies. We compare weathering records from the Pearl River delta of southern China and Ocean Drilling Program (ODP) Sites 1144 and 1146 on the northeastern slope of the South China Sea in order to test which proxies are the most widely applicable and robust. Comparison with speleothem rainfall records indicates that K/Al tracks precipitation variations most closely and out-performs the widely used Chemical Index of Alteration (CIA). Correlation of K/Al and kaolinite/illite indicates that this clay ratio is also an effective proxy of weathering intensity across all sites and timescales. Kaolinite/smectite, and to a lesser extent smectite/(illite+chlorite), are also indicative of weathering intensity, but show more scatter between sites that may be linked to provenance effects. Mg/Al is relatively immune to grain-size effects, but does not correlate well with other proxies. K/Rb is a reasonably reliable indicator of chemical weathering intensity and may be more sensitive than CIA or K/Al to weathering changes over short timescales and when weathering is not too intense. 87 Sr/ 86 Sr can be useful but can be influenced by both grain size and provenance effects. In general marine archives of fluvial sediment may record variations in weathering linked to climate, but these are increasingly signals of reworking going further offshore.

Description: Reconstructing variations in the intensity of chemical weathering in river basins is crucial if we are to understand how climate change impacts environment and whether there are feedbacks between climate and weathering processes. Quantifying chemical weathering is, however, a complicated process, involving a number of competing proxies. We compare weathering records from the Pearl River delta of southern China and Ocean Drilling Program (ODP) Sites 1144 and 1146 on the northeastern slope of the South China Sea in order to test which proxies are the most widely applicable and robust. Comparison with speleothem rainfall records indicates that K/Al tracks precipitation variations most closely and out-performs the widely used Chemical Index of Alteration (CIA). Correlation of K/Al and kaolinite/illite indicates that this clay ratio is also an effective proxy of weathering intensity across all sites and timescales. Kaolinite/smectite, and to a lesser extent smectite/(illite+chlorite), are also indicative of weathering intensity, but show more scatter between sites that may be linked to provenance effects. Mg/Al is relatively immune to grain-size effects, but does not correlate well with other proxies. K/Rb is a reasonably reliable indicator of chemical weathering intensity and may be more sensitive than CIA or K/Al to weathering changes over short timescales and when weathering is not too intense. 87 Sr/ 86 Sr can be useful but can be influenced by both grain size and provenance effects. In general marine archives of fluvial sediment may record variations in weathering linked to climate, but these are increasingly signals of reworking going further offshore.