Compilation of 231Protactinium/230 Thorium excess ratios in various surface sediment samples from the South Atlantic

In large areas of the world's oceans, there is a relationship between the mass flux of particulate matter and the unsupported 231Pa/230Th (xs231Pa/xs230Th) activity ratio of recent sediments. This observation forms the basis for using the xs231Pa/xs230Th ratio as a proxy for past changes in export productivity. However, a simple relationship between xs231Pa/xs 230Th ratio and particle flux requires that the water residence time in an ocean basin is far in excess of the scavenging residence time of 231Pa, and that the composition of sinking particles maintains a strong preference for the adsorption of 230Th over 231Pa with a constant 230Th/231Pa fractionation factor (F). The best correlation between xs231Pa/xs230Th ratio and mass flux is found in the Pacific Ocean. In the Atlantic, the contrast in the xs231Pa/xs230Th ratios between open ocean (low flux regions) and ocean margins (high flux regions) is much less pronounced due to the shorter residence time of deep water, resulting in less effective boundary scavenging of 231Pa. In the Southern Ocean, south of the Polar Front, there is no more a simple relationship between xs231Pa/xs230Th and particle flux. This is a result of a southward decrease in F, probably reflecting the increased opal content of sinking particles. Opal does not fractionate 231Pa and 230Th significantly. This lack of fractionation results in high xs231Pa/xs230Th ratios in opal-dominated regions, even in areas of very low particle fluxes such as the Weddell Sea. The xs231Pa/xs230Th ratio can therefore only be used as a paleoproductivity proxy if, in the time interval of interest, changes in the basin ventilation rate and differential scavenging of both radionuclides due to changes in the chemical composition of particulate matter can be excluded.

Geochemical data derived from an absolutely dated shell-based chronology for the Irish Sea

The identification in various proxy records of periods of rapid (decadal scale) climate change over recent millennia, together with the possibility that feedback mechanisms may amplify climate system responses to increasing atmospheric CO2, highlights the importance of a detailed understanding, at high spatial and temporal resolutions, of forcings and feedbacks within the system. Such an understanding has hitherto been limited because the temperate marine environment has lacked an absolute timescale of the kind provided by tree-rings for the terrestrial environment and by corals for the tropical marine environment. Here we present the first annually resolved, multi-centennial (489-year), absolutely dated, shell-based marine master chronology. The chronology has been constructed by detrending and averaging annual growth increment widths in the shells of multiple specimens of the very long-lived bivalve mollusc Arctica islandica, collected from sites to the south and west of the Isle of Man in the Irish Sea. The strength of the common environmental signal expressed in the chronology is fully comparable with equivalent statistics for tree-ring chronologies. Analysis of the 14C signal in the shells shows no trend in the marine radiocarbon reservoir correction (DR), although it may be more variable before ~1750. The d13C signal shows a very significant (R**2 = 0.456, p < 0.0001) trend due to the 13C Suess effect.