Proxy For Past Changes Research Articles

The 231Pa/ 230Th ratio as a proxy for past changes in opal fluxes in the Indian sector of the Southern Ocean

Published scavenging models generally assume that the 231Pa/ 230Th ratios of surface sediments are primarily determined by the mass flux of particles. In this study we compare the 230Th normalized vertical fluxes of both total sediments and opal to the 231Pa/ 230Th ratios measured from six sediment cores sampled across the Antarctic Circumpolar Current (ACC). We observe a better correlation between the 231Pa/ 230Th ratios and the vertical opal fluxes corrected for dissolution than with total sediment vertical fluxes. This observation indicates that opal may explain the enhanced scavenging of 231Pa. This result is consistent with the studies of Loeff and Berger [Deep-Sea Res. 40 (2) (1993) 339], Kumar et al. [Nature 378 (1995) 675] and Walter et al. [Earth Planet. Sci. Lett. 149 (1997) 85] who speculated that opal may explain enhanced scavenging of 231Pa. Finally, our results suggest that the 231Pa/ 230Th ratio is a reliable indicator of opal mass flux and can be used, taking some precautions, as a proxy for opal paleoproductivity in the Indian sector of the Southern Ocean.

Quantifying carbonate dissolution and calibration of foraminiferal dissolution indices in the Somali Basin

Two sediment traps moored off Somalia in 1992–1993 collected similar settling fluxes of carbonate and siliceous shells formed by various plankton groups. Planktic foraminifera showed large seasonal variations, with more than 74% of the total planktic foraminifera flux collected during the SW monsoon (summer upwelling), when Globigerina bulloides was dominating along with Globigerinita glutinata and Neogloboquadrina dutertrei. The intermonsoon and NE monsoon assemblages were dominated by Globigerinoides ruber. We used the trap records as our ‘no dissolution’ reference for comparison with three boxcore recorders in order to quantify the carbonate dissolution along a depth transect. Dissolution increases downslope, from Station 905 to 907 and 915 at depths of 1567 m, 2807 m and 4035 m, respectively. The carbonate fraction of the sediment at Station 915, which is located near the CCD, is the most affected by dissolution, with more than 97% of the planktic foraminifera dissolved. Here, the planktic foraminifera assemblage is strongly modified, with thick walled species such as N. dutertrei, Globorotalia tumida and Pulleniatina obliquiloculata as the most resistant. It is not representative of the settling assemblage. The planktic foraminiferal assemblages of the sediment surface at Stations 905 and 907 remain similar to the trap assemblages and the foraminifera are well preserved, although only 25% of foraminifera are apparently preserved at Station 905 and 8% at Station 907. Those numbers are surprisingly low and infer that only a small fraction of the foraminiferal carbonate production is buried and removed from the carbon cycle. This discrepancy between the export and buried flux is partly be due to (bio)mechanical destruction by benthic processes and to supralysoclinal dissolution, due to metabolic CO 2 generated by the benthic organisms. Another important factor is the interannual variability of the productivity, well known in the Arabian Sea. The calibration of commonly used foraminiferal dissolution indices (percentage of foraminiferal fragments, percentage of resistant species, foraminiferal dissolution index (FDX)) to our data only shows reliable results for high dissolution levels (>97%). Off Somalia the most accurate of the proxies is the percentage of foraminiferal fragments compared to the other methods tested, i.e., FDX, planktic foraminiferal loss (L), percentage of radiolarians and diatoms, percentage of benthic foraminifera. The species assemblage appears to be not significantly modified by dissolution unless the estimated shell loss is high, >92% of the arriving shells in our samples. This level is expressed in the percentage of fragmentation, the percentage of radiolarians and diatoms, the percentage of resistant foraminifera species, and FDX as 80%, 35%, 25% and 1.8, respectively. The relative abundance of Globigerina bulloides is a valid SW monsoon/upwelling proxy only when dissolution is moderate (<92%). Globigerina bulloides and Globigerinoides ruber have similar burial efficiencies and susceptibilities to dissolution. Thus, the ratio G. bulloides/ G. ruber is a valid proxy for past changes in the intensity of the SW monsoon even in the strongly dissolved samples. In our sediment record the ratio G. bulloides/ G. ruber indicates that the SW monsoon was stronger in the recent past than in 1992–93.

Fluxes of planktic foraminifera in response to monsoonal upwelling on the Somalia Basin margin

Sediment trap samples collected off Somalia (bi)weekly from early June 1992 through mid February 1993 show large seasonal variations in the shell flux and species composition of planktic foraminifera. These variations mirror the monsoon-driven circulation, which resulted in massive upwelling and offshore eddy transport in summer during the SW Monsoon (June–September) and accounted for nearly 90% of the estimated annual shell flux of planktic foraminifera. G. bulloides dominated the SW Monsoon along with G. glutinata, T. quinqueloba, N. dutertrei and T. iota in a fauna-rich species but with a low diversity and equitability. During the autumn intermonsoon (November–December) the ocean became stratified and the nutrient exhausted, whereas during the NE Monsoon (January–March) a warm surface mixed layer developed with the nutrients entrained by deep wind mixing. It was dominated by G. ruber in association with G. tenella, G. aequilateralis, G. trilobus/ G. sacculifer and G. menardii in a fauna poorer in species but with a higher diversity and equitability. Consequently, the ratio in the abundance of G. bulloides and G. ruber follows the monsoonal cycle closely, as both species persist throughout the year. Only the rare G. theyeri seems to be restricted to the SW Monsoon. In October at the end of the SW Monsoon, the trap at 265 m above the bottom intercepted massive amounts of fine-grained carbonate (<125 μm) resuspended from the shelf and upper slope. It contained extreme fluxes of bioclastic fragments, benthic foraminifera and benthic ostracods as well as enhanced fluxes of small-sized planktic foraminifera <125 μm , notably G. rubescens. Integrated over the year, the species composition intercepted by the trap shows a very good match with the shell fauna in a core top sediment from the same site. Apparently, the year-estimated monsoonal record of planktic foraminifera, which is strongly dominated by the SW Monsoon production, survives burial intact. Consequently, the species composition of planktic foraminifera and measures such as the bulloides/ ruber ratio can be applied as a proxy for past changes in the intensity of the SW Monsoon off Somalia compared to the modern condition.

Latitudinal δ 13C org variations in sinking matter and sediments from the South Atlantic: effects of anthropogenic CO 2 and implications for paleo-PCO 2 reconstructions

Latitudinal variations of δ 13C org of plankton, sinking particles and surface sediments of the southern Atlantic Ocean have been compiled and compared to literature-derived [CO 2 (aq)] values in surface waters. We observed less variability of the sediment-δ 13C data at a given latitude compared to the plankton values which more sensitively record seasonal changes. In the tropical–subtropical Atlantic with low seasonality, we measured a relatively small latitudinal variation (−18.5 to −22.5‰) of the sediment-δ 13C. In contrast, a steep gradient of both isotope values and [CO 2 (aq)] was found south of 40°S. South of 60°S, the sediment-δ 13C values were between −23 and −26.5‰ corresponding to generally higher [CO 2 (aq)] values. The surface sediments were generally heavier by 0.5–3.2‰ in the tropical–subtropical Atlantic and 3.0–4.6‰ in Southern Ocean, respectively, compared to sinking matter; they were also heavier by up to 3.2‰ than freshly sedimented phytodetritus sampled in the Southern Ocean. These differences may at least be partly attributed to the anthropogenic PCO 2 increase leading to an enhanced effect in the Southern Ocean due to increasing CO 2 solubility. We estimated the preindustrial [CO 2 (aq)] from sediment-δ 13C org to be lower by 1–2 μmol l −1 in warm waters and up to 8 μmol l −1 in cold waters compared to present day measurements. We found that the isotope values of the surface sediments were generally linked to surface water [CO 2 (aq)] (preindustrial), resulting in a relationship rather similar to that derived from plankton data [Rau, G.H., 1994. Variations in sedimentary organic δ 13C as a proxy for past changes in ocean and atmospheric CO 2 concentrations. In: Zahn, R., Pedersen, T.F., Kaminski, M.A., Labeyrie, L. (Eds.), Carbon Cycling in the Glacial Ocean, NATO Asi Series, Global Environmental Change, Vol. 17]. However, estimations of past [CO 2 (aq)] from sediment-δ 13C org may have a large uncertainty especially in the Southern Ocean. Here, our data show the weakest correlation, probably due to effects of changing growth rates and productivity, in particular at the frontal zones.

231Pa/230Th ratios in sediments as a proxy for past changes in Southern Ocean productivity

THE biological productivity of the oceans is sensitive to changes in climate, which can affect essential factors such as nutrient and light availability. In turn, ocean productivity may influence climate by regulating the partitioning of carbon dioxide, a greenhouse gas, between the ocean and the atmosphere. Investigators have attempted to link variations in atmospheric CO2 content, recorded in ice cores1,2, to the productivity of the Southern Ocean3–6, but an unambiguous means of assessing past changes in ocean productivity has been lacking. Here we exploit established relationships between 231Pa/230Th ratios and particle flux7–12 to infer, from the analysis of dated sediment cores, variability through time of fluxes of particulate biogenic material exported from surface waters. Records from two cores in the Atlantic sector of the Southern Ocean indicate that ocean productivity during glacial periods was lower than at present south of the Antarctic polar front, and support earlier conclusions13–16 that the zone of maximum productivity migrated northwards during glacial conditions. Although further work at other sites is needed for an assessment of changes in total Antarctic productivity, our technique has the potential to provide this information while avoiding some of the limitations of other productivity proxies.

Paleosalinity from trace metals in fossil ostracodes compared with observational records at Devils Lake, North Dakota, USA

The chemistry of closed-basin lakes responds directly to the hydrologic budget through evaporative concentration of dissolved salts, and the sediments of these basins contain several excellent fossil and geochemical proxies for past changes in water chemistry and salinity. The trace-metal content of ostracode shells, among the most promising of these paleosalinity methods, is further explored in this study of Mg and Sr partitioning in Candona rawsoni, a widespread ostracode of the North American Great Plains. The molar distribution coefficients K D [Mg] and K D [Sr] for this species, K D<[Mg]= (Mg/Ca) CaCO 3−0.004 (Mg 2+/Ca 2+)H 2O =9.68×10 −5×T( °C) K D[Sr]= (Sr/Ca) CaCO 3 (Sr 2+/Ca 2+)H 2Oz =0.406 are determined from laboratory experiments in which C. rawsoni are cultured under controlled temperatures and salinities. Our formulation of K D [Mg] contains a correction term for excess Mg that is incorporated into the shell during early calcification. The accuracy of the distribution coefficients is tested against field collections of live ostracodes from a suite of 12 Dakota lakes. The K D -inferred values for (Mg 2+/Ca 2+) H 2O and (Sr 2+/Ca 2+) H 2O are in close accord with the same ratios measured in the lakes. A comparison of ostracode-reconstructed and historically measured salinities for Devils Lake, North Dakota, shows good agreement within the salinity range preferred by C. rawsoni (1–10% 0), although salinity excursions beyond this optimum are poorly represented in the sedimentary record. In most lakes salinity is more readily inferred from (Mg/Ca) H 2O than (Sr/Ca) H 2O because Sr solubility is a complex function of several carbonate phases. As shown for Devils Lake, (Sr/Ca) H 2O may not vary systematically with the salinity of some brines.