Sea surface reservoir ages must be known to establish a common chronological framework for marine, continental, and cryospheric paleoproxies, and are crucial for understanding ocean-continent climatic relationships and the paleoventilation of the ocean. Radiocarbon dates of planktonic foraminifera and tephra contemporaneously deposited over Mediterranean marine and terrestrial regions reveal that the reservoir ages were similar to the modern one (approximately 400 years) during most of the past 18,000 carbon-14 years. However, reservoir ages increased by a factor of 2 at the beginning of the last deglaciation. This is attributed to changes of the North Atlantic thermohaline circulation during the massive ice discharge event Heinrich 1.

We present late Quaternary records of metastable carbonate dissolution determined for sediment cores recovered from intermediate water depths on the Nicaragua Rise (Caribbean Sea, 1000–1894 m) and near the Bahama Banks (western North Atlantic Ocean, 655 and 1934 m). Upper North Atlantic Deep Water is believed to dominate these two regions at present. Both areas are predicted to be good locations to study past variations of average middepth Atlantic chemistry and circulation. However, statistical analyses of metastable carbonate dissolution indices (% Mg calcite, pteropod abundance, % whole pteropods, and % clear pteropods) yielded a composite dissolution index (CDI) which displays different carbonate dissolution histories for Bahama and Nicaragua Rise sediments. The Bahama records resemble deep Atlantic carbonate records with dissolution during glacial oxygen isotope stages 6 and 4 and with preservation during interglacial stages 5 and 1. We observe two dissolution patterns at intermediate water depths of the Caribbean. The “deep” intermediate water pattern (1200–1894 m) resembles deep Caribbean carbonate records with dissolution during interglacial periods and preservation during glacial intervals. The “shallow” intermediate water record (<1200 m) resembles at times the Bahamas record and at other times the deep Caribbean record. These CDI records suggest that different water masses occupied the intermediate depth western North Atlantic and Caribbean during much of the late Quaternary. Observed differences between these regions may be related to variations in the flow of nutrient‐rich, low CO3= Antarctic Intermediate Water through the Caribbean or to changes in the ventilation rate of equatorial thermocline and intermediate waters. These water‐mass variations have influenced Caribbean carbon chemistry from the base of the thermocline down to abyssal water depths and may have had a significant effect on North Atlantic circulation and nutrients.

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Ocean Drilling Program (ODP) Leg 115 post-cruise research was focused on two Maldives sites, more precisely on the top 108 m of Hole 716B (water depth, 540 m), equivalent to the past 3.5 m.y., and the top 19.5 m of Hole 714A (water depth, 2195 m), equivalent to the past 0.55 m.y. These sediments consist of mostly unaltered and undisturbed, turbidite-free, periplatform ooze. Results of our research are compared with existing data on Hole 633A (water depth, 1681 m), drilled in the Bahamas during ODP Leg 101, using age/depth models built on the basis of oxygen isotope, nannofossil, and magnetic stratigraphies. Climate-induced, long-term (roughly 0.5 m.y.) aragonite cycles, superposed on short-term (roughly 0.04 and 0.1 m.y.) aragonite cycles, have been established at least during the past 2.0 m.y., in the Maldives and the Bahamas. Our most interesting result is the clear correlation among the aragonite long-term cycles in the Maldives and the Bahamas and the carbonate-preservation, long-term cycles from the open Pacific, Indian, and North Atlantic oceans. The mid-Brunhes dissolution interval, corresponding to the youngest preservation minima of the carbonate-preservation, longterm cycles, is clearly defined by fine aragonite minimum values in the deep periplatform sites, and by maximum fragmentation of pteropod tests in the shallow sites. Aragonite and planktonic 5 18 O records, usually in phase during the late Pleistocene, display, further back in time, discreet intervals where the two records do not match with one another. Major mismatches between both records occur synchronously in the Maldives and Bahamas periplatform sites and seem to correspond to extreme events of either carbonate-preservation or dissolution in the deep pelagic carbonate sites of the equatorial Pacific Ocean. Based on our findings, short-and long-term aragonite cycles can no longer be explained only by variations of aragonite input from the nearby shallow carbonate banks, in response to their alternate flooding and exposure through cyclic sea-level fluctuations. The aragonite long-term cycles in the periplatform environments are interpreted as carbonatepreservation cycles at intermediate-water depths. Their occurrence shows, therefore, that the carbonate chemistry of the entire water column has been influenced by long-term (0.5 m.y.) cyclic variations during the past 2.0 m.y. These major changes of the water-column carbonate chemistry are linked to the climate-induced carbon cycling among the different atmospheric, oceanic, and sedimentary carbon reservoirs.

The geometry, timing, and rate of fluid-flow through carbonate margins and platforms is not well constrained. In this study, we use U concentrations and isotope ratios measured on small volumes of pore-water from Bahamas slope sediment, coupled with existing chlorinity data, to place constraints on the fluid-flow in this region and, by implication, other carbonate platforms. These data also allow an assessment of the behaviour of U isotopes in an unusually well constrained water-rock system. We report pore-water U concentrations which are controlled by dissolution of high-U organic material at shallow depths in the sediment and by reduction of U to its insoluble 4C state at greater depths. The dominant process influencing pore-water ( 234 U= 238 U) is alpha recoil. In Holocene sediments, the increase of pore-water ( 234 U= 238 U) due to recoil provides an estimate of the horizontal flow rate of 11 cm=year, but with considerable uncertainty. At depths in the sediment where conditions are reducing, features in the U concentration and ( 234 U= 238 U) profiles are offset from one another which constrains the effective diffusivity for U in these sediments to be ³1-2 ð 10 8 cm 2 s 1 . At depths between the Holocene and these reducing sediments, pore-water ( 234 U= 238 U) values are unusually low due to a recent increase in the dissolution rate of grain surfaces. This suggests a strengthening of fluid flow, probably due to the flooding of the banks at the last deglaciation and the re-initiation of thermally-driven venting of fluid on the bank top and accompanying recharge on the slopes. Interpretation of existing chlorinity data, in the light of this change in flow rate, constrain the recent horizontal flow rate to be 10.6 (š3.4) cm=year. Estimates of flow rate from ( 234 U= 238 U) and Cl are therefore in agreement and suggest flow rates close to those predicted by thermally-driven models of fluid flow. This agreement supports the idea that flow within the Bahamas Banks is mostly thermally driven and suggests that flow rates on the order of 10 cm=year are typical for carbonate platforms where such flow occurs. © 1999 Elsevier Science B.V. All rights reserved. Henderson et al. / Earth and Planetary Science Letters 169 (1999) [99][100][101][102][103][104][105][106][107][108][109][110][111]