Glacial to Holocene changes in carbonate and clay sedimentation in the equatorial Pacific Ocean estimated from thorium 230 profiles

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Abstract
Uranium series nuclide concentrations have been measured on sediments from five box cores from an equatorial Pacific transect. 230Thexcess activities show discontinuities at the Holocene‐glacial boundary as dated by 14C. The glacial sedimentation rates determined by 230Th and 14C are 2.5‐3.0 cm/kyr. The Holocene rates from 230Th are much lower than those dated by 14C (1.9–2.3 cm/kyr) because of carbonate dissolution. 230Th sedimentation fluxes exceed water column supply by factors of 1.2–1.8 in the Holocene and 1.8–3.0 in the glacial sections. A number of models have been applied to calculate carbonate dissolution rates. The results show that carbonate dissolution rates in the Holocene (in g/cm²kyr) equal 1.5 × 10−3 exp (1.4D) where D is water depth in kilometers. A point‐by‐point estimation of sediment fluxes through time show that clay accumulation rates in the area have been near constant at 0.1–0.2 g/cm²kyr over the past 20 kyr whereas carbonate accumulation rates have decreased dramatically from 0.6–1.0 g/cm²kyr in the glacial sections of the cores to 0.2–0.6 g/cm²kyr in the Holocene. The errors caused by the uncertainties in the age of the termination of the last glacial period have been investigated and results show that a range of 11–14 kyr leads to an error upper limit of about 30% in the estimation of CaCO3 dissolution rates. The response time of CaCO3 and 230Thex concentrations in the mixed layer of sediments due to an impulse of change in CaCO3 dissolution rate has also been discussed, showing that the observed changes in carbonate dissolution may be explained in terms of a single or a continuous change, depending upon the thickness of the mixed layer.