149 resultados para Mg-al Alloys


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We studied magnesium:calcium (Mg/Ca) ratios in shells of the deep-sea ostracode genus Krithe from a short interval in the middle Pliocene between 3.29 and 2.97 Ma using deep-sea drilling sites in the North and South Atlantic in order to estimate bottom water temperatures (BWT) during a period of climatic warmth. Results from DSDP and ODP Sites 552A, 610A, 607, 658A, 659A, 661A and 704 for the period Ma reveal both depth and latitudinal gradients of mean Mg/Ca values. Shallower sites (552A, 610A and 607) have higher mean Mg/Ca ratios (10.3, 9.7, 10.1 mmol/mol) than deeper sites (661A, 6.3 mmol/mol), and high latitude North Atlantic sites (552A, 610 and 607) have higher Mg/Ca ratios than low latitude (658A: 9.8 mmol/mol, 659A: 7.7 mmol/mol, 661A: 6.3 mmol/mol) and Southern Ocean (704: 8.0 mmol/mol) sites. Converting Mg/Ca ratios into estimated temperatures using the calibration of Dwyer et al. (1995) [Dwyer, G.S., Cronin, T.M., Baker, P.A., Raymo, M.E., Buzas, J.S., Corrège, T., 1995. North Atlantic deepwater temperature change during late Pliocene and late Quaternary climatic cycles. Science 270, 1347-1351] suggests that mean middle Pliocene bottom water temperatures at the study sites in the deep Atlantic were about the same as modern temperatures. However, brief pulses of elevated BWT occurred several times between 3.29 and 2.97 Ma in both the North and South Atlantic Ocean suggesting short-term changes in deep ocean circulation.

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This study focuses on the vertical distribution of authigenic carbonates (aragonite and high Mg-calcite) in the form of finely disseminated precipitates as well as massive carbonate concretions present in and above gas hydrate bearing sediments of the Northern Congo Fan. Analyses of Ca, Mg, Sr and Ba in pore water, bulk sediments and authigenic carbonates were carried out on gravity cores taken from three pockmark structures (Hydrate Hole, Black Hole and Worm Hole). In addition, a background core was retrieved from an area not influenced by fluid seepage. Pore water Sr/Ca and Mg/Ca ratios are used to reveal the current depths of carbonate formation as well as the mineralogy of the authigenic precipitates. The Sr/Ca and Mg/Ca ratios of bulk sediments and massive carbonate concretions were applied to infer the presence and depth distribution of authigenic aragonite and high Mg-calcite, based on the approach presented by Bayon et al. [Bayon et al. (2007). Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments. Marine Geology 241(1-4), 93-109, doi:10.1016/j.margeo.2007.03.007]. We show that the approach developed by Bayon et al. (2007) for sediments of cold seeps of the Niger Delta is also suitable to identify the mineralogy of authigenic carbonates in pockmark sediments of the Congo Deep-Sea Fan. We expand this approach by combining interstitial with solid phase Sr/Ca and Mg/Ca ratios, which demonstrate that high Mg-calcite is the predominant authigenic carbonate that currently forms at the sulfate/methane reaction zone (SMRZ). This is the first study which investigates both solid phase and pore water signatures typical for either aragonite or high Mg-calcite precipitation for the same sediment cores and thus is able to identify active and fossil carbonate precipitation events. At all investigated pockmark sites fossil horizons of the SMRZ were deduced from high Mg-calcite located above and below the current depths of the SMRZ. Additionally, aragonite enrichments typical for high seepage rates were detected close to the sediment surface at these sites. However, active precipitation of aragonite as indicated by pore water characteristics only occurs at the Black Hole site. Dissolved and solid phase Ba concentrations were used to estimate the time the SMRZ was fixed at the current depths of the diagenetic barite fronts. The combined pore water and solid phase elemental ratios (Mg/Ca, Sr/Ca) and Ba concentrations allow the reconstruction of past changes in methane seepage at the investigated pockmark sites. At the Hydrate Hole and Worm Hole sites the time of high methane seepage was estimated to have ceased at least 600 yr BP. In contrast, a more recent change from a high flux to a more dormant stage must have occurred at the Black Hole site as evidenced by active aragonite precipitation at the sediment surface and a lack of diagenetic Ba enrichments.

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The glacial marine isotope stage 14 (MIS 14) appears in many climate records as an unusually warm glacial. During this period an almost monospecific, up to 1.5 m thick, laminated layer of the giant diatom Ethmodiscus rex has been deposited below the South Atlantic Subtropical Gyre. This oligotrophic region is today less favorable for diatom growth with sediments typically consisting of calcareous nannofossil oozes. We have reconstructed temperatures and the stable oxygen isotopic compositions of sea surface and thermocline water (d18Ow) from planktonic foraminiferal (Globigerinoides ruber and Globorotalia inflata) Mg/Ca and stable oxygen isotopes to test whether perturbations in surface ocean conditions contributed to the deposition of the diatom layer at ~530 kyr B.P. Temperatures and d18Ow values reconstructed from this diatom ooze interval are highly variable, with maxima similar to interglacial values. Since the area of the Ethmodiscus oozes resembles the region where Agulhas rings are present, we interpret these hydrographic changes to reflect the varying influence of warm and saline water of Indian Ocean origin that entered the Subtropical Gyre trapped in Agulhas rings. The formation of the Ethmodiscus oozes is associated with a period of maximum Agulhas leakage and a maximum frequency of Agulhas ring formation caused by a termination-type position of the Subtropical Front during the unusual warm MIS 14. The input of silica through the Agulhas rings enabled the shift in primary production from calcareous nannoplankton to diatoms, leading to the deposition of the massive diatom oozes.

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The importance of intermediate water masses in climate change and ocean circulation has been emphasized recently. In particular, Southern Ocean Intermediate Waters (SOIW), such as Antarctic Intermediate Water and Subantarctic Mode Water, are thought to have acted as active interhemispheric transmitter of climate anomalies. Here we reconstruct changes in SOIW signature and spatial and temporal evolution based on a 40 kyr time series of oxygen and carbon isotopes as well as planktic Mg/Ca based thermometry from Site GeoB12615-4 in the western Indian Ocean. Our data suggest that SOIW transmitted Antarctic temperature trends to the equatorial Indian Ocean via the "oceanic tunnel" mechanism. Moreover, our results reveal that deglacial SOIW carried a signature of aged Southern Ocean deep water. We find no evidence of increased formation of intermediate waters during the deglaciation.

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An early Holocene record from the Gulf of Mexico (GOM) reveals climatic and hydrologic changes during the interval from 10.5 to 7 thousand calendar years before present from paired analyses of Mg/Ca and d18O on foraminiferal calcite. The sea surface temperature record based on foraminiferal Mg/Ca contains six oscillations and an overall ~1.5°C warming that appears to be similar to the September-March insolation difference. The d18O of seawater in the GOM (d18OGOM) record contains six oscillations, including a -0.8 per mil excursion that may be associated with the "8.2 ka climate event" or a broader climate anomaly. Faunal census records from three GOM cores exhibit similar changes, suggesting subcentennial-scale variability in the incursions of Caribbean waters into the GOM. Overall, our results provide evidence that the subtropics were characterized by decadal- to centennial-scale climatic and hydrologic variability during the early Holocene.

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Shell chemistry of planktic foraminifera and the alkenone unsaturation index in 69 surface sediment samples in the tropical eastern Indian Ocean off West and South Indonesia were studied. Results were compared to modern hydrographic data in order to assess how modern environmental conditions are preserved in sedimentary record, and to determine the best possible proxies to reconstruct seasonality, thermal gradient and upper water column characteristics in this part of the world ocean. Our results imply that alkenone-derived temperatures record annual mean temperatures in the study area. However, this finding might be an artifact due to the temperature limitation of this proxy above 28°C. Combined study of shell stable oxygen isotope and Mg/Ca ratio of planktic foraminifera suggests that Globigerinoides ruber sensu stricto (s.s.), G. ruber sensu lato (s.l.), and G. sacculifer calcify within the mixed-layer between 20 m and 50 m, whereas Globigerina bulloides records mixed-layer conditions at ~50 m depth during boreal summer. Mean calcifications of Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, and Globorotalia tumida occur at the top of the thermocline during boreal summer, at ~75 m, 75-100 m, and 100 m, respectively. Shell Mg/Ca ratios of all species show a significant correlation with temperature at their apparent calcification depths and validate the application of previously published temperature calibrations, except for G. tumida that requires a regional Mg/Ca-temperature calibration (Mg/Ca = 0.41 exp (0.068*T)). We show that the difference in Mg/Ca-temperatures of the mixed-layer species and the thermocline species, particularly between G. ruber s.s. (or s.l.) and P. obliquiloculata, can be applied to track changes in the upper water column stratification. Our results provide critical tools for reconstructing past changes in the hydrography of the study area and their relation to monsoon, El Niño-Southern Oscillation, and the Indian Ocean Dipole Mode.

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During the early Pliocene warm period (~4.6-4.2 Ma) in the Eastern Equatorial Pacific upwelling region, sea surface temperatures were warm in comparison to modern conditions. Warm upwelling regions have global effects on the heat budget and atmospheric circulation, and are argued to have contributed to Pliocene warmth. Though warm upwelling regions could be explained by weak winds and/or a deep thermocline, the temporal and spatial evolution of the equatorial thermocline is poorly understood. Here we reconstruct temporal and spatial changes in subsurface temperature to monitor thermocline depth and show the thermocline was deeper during the early Pliocene warm period than it is today. We measured subsurface temperature records from Eastern Equatorial Pacific ODP transect Sites 848, 849, and 853 using Mg/Ca records from Globorotalia tumida, which has a depth habitat of ~50-100 m. In the early Pliocene, subsurface temperatures were ~4-5°C warmer than modern temperatures, indicating the thermocline was relatively deep. Subsurface temperatures steeply cooled ~2-3°C from 4.8 to 4.0 Ma and continued to cool an additional 2-3°C from 4.0 Ma to present. Compared to records from other regions, the data suggests the pronounced subsurface cooling between 4.8 and 4.0 Ma was a regional signal related to restriction of the Isthmus of Panama, while continued cooling from 4.0 Ma to present was likely related to global processes that changed global thermocline structure. Additionally, the spatial evolution of the equatorial thermocline along a N-S transect across ODP Sites 853, 849 and 848 suggests an intensification of the southeast trades from the Pliocene to present. Large-scale atmospheric and oceanographic circulation processes link high and low latitude climate through their influence on equatorial thermocline source water regions and consequently the equatorial thermocline. Through these low latitude/high latitude linkages, changes in the equatorial thermocline and thermocline source water played an important role in the transition from the warm Pliocene to the cold Pleistocene.

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The evolution of productivity, redox conditions, temperature, and ventilation during the deposition of an Aegean sapropel (S1) is independently constrained using bulk sediment composition and high-resolution single specimen benthic foraminiferal trace metal and stable isotope data. The occurrence of benthic foraminifer, Hoeglundina elegans (H. elegans), through a shallow water (260 m) sapropel, permits for the first time a comparison between dissolved and particulate concentrations of Ba and Mn and the construction of a Mg/Ca-based temperature record through sapropel S1. The simultaneous increase in sedimentary Ba and incorporated Ba in foraminiferal test carbonate, (Ba/Ca)H. elegans, points to a close coupling between Ba cycling and export productivity. During sapropel deposition, sedimentary Mn content ((Mn/Al)sed) is reduced, corresponding to enhanced Mn2+ mobilization from sedimentary Mn oxides under suboxic conditions. The consequently elevated dissolved Mn2+ concentrations are reflected in enhanced (Mn/Ca)H. elegans levels. The magnitude and duration of the sapropel interruption and other short-term cooling events are constrained using Mg/Ca thermometry. Based on integrating productivity and ventilation records with the temperature record, we propose a two-mode hysteresis model for sapropel formation.