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We have analyzed the stable oxygen isotopic composition of two Porites corals from the Chagos Archipelago, which is situated in the geographical center of the Indian Ocean. Coral δ18O at this site reliably records temporal variations in precipitation associated with the Intertropical Convergence Zone (ITCZ). Precipitation maxima occur in boreal winter, when the ITCZ forms a narrow band across the Indian Ocean. The Chagos then lies within the center of the ITCZ, and rainfall is strongly depleted in δ18O. A 120-yr coral isotopic record indicates an alternation of wet and dry intervals lasting 15 to 20 yr. The most recent 2 decades are dominated by interannual variability, which is tightly coupled to the El Niño-Southern Oscillation (ENSO). This is unprecedented in the 120 yr of coral record. As the ITCZ is governed by atmospheric dynamics, this provides evidence of a major change in the coupled ENSO-monsoon system. 相似文献
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C. J. R. Braithwaite L. F. Montaggioni G. F. Camoin H. Dalmasso W. C. Dullo A. Mangini 《International Journal of Earth Sciences》2000,89(2):431-445
Until recently, concepts of coral reef growth and accumulation have been predominantly based on a Darwinian model. In this,
the upwards and outwards growth of a reef core (a coral framework) takes place over a foreslope consisting of reef talus,
with the simultaneous filling of the back-reef lagoon by reef-derived debris. The principal adaptations of this pattern relate
to the influence of relative changes in sea level and commonly ignore oceanographic factors such as storm frequency and severity.
Boreholes through the outer edge of a fringing reef in the Seychelles, western Indian Ocean, reveal a record of Holocene sediment
accumulation first established approximately 8 ka ago. Faunal and floral associations show that growth of this body began
in relatively deep water but that this shallowed to <5 m within 1 ka. Subsequent accumulation was of “keep-up” style but,
as the rate of sea-level rise slowed, shoaling became more frequent and aggradation was limited by reducing accommodation
space. Constructional facies are characterised either by massive corals, including Leptastrea, Porites and faviids, or by branching corals, typically Acropora of the danai-robusta group. Coral surfaces may be encrusted by red algae, foraminifera and vermetids, and are commonly bored by filamentous algae,
clionids and molluscs. However, detrital facies are volumetrically dominant, and the paucity of a constructional framework
requires a re-evaluation of models of reef accretion. New models relate the geometry of accretion to the interplay between
extreme storm events and fairweather hydrodynamic conditions. These suggest that a contiguous framework forms in areas of
moderate fairweather energy without extreme storm events. Severe storms destroy the continuity of reef structures and generate
increasing volumes of coarse detritus. Low storm severity, coupled with low fairweather hydrodynamic energy, may promote the
accumulation of fine-grained reef-derived sediments that inhibit framework growth. While ecology reflects year-by-year sea
conditions, lithology and structure are controlled by exceptional storms, with the effects of changing sea level superimposed.
Received: 30 November 1998 / Accepted: 4 November 1999 相似文献
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F. Wombacher A. Eisenhauer N. Gussone M. Regenberg W.-Chr. Dullo 《Geochimica et cosmochimica acta》2011,75(19):5797-253
This survey of magnesium stable isotope compositions in marine biogenic aragonite and calcite includes samples from corals, sclerosponges, benthic porcelaneous and planktonic perforate foraminifera, coccolith oozes, red algae, and an echinoid and brachiopod test. The analyses were carried out using MC-ICP-MS with an external repeatability of ±0.22‰ (2SD for δ26Mg; n = 37), obtained from a coral reference sample (JCp-1).Magnesium isotope fractionation in calcitic corals and sclerosponges agrees with published data for calcitic speleothems with an average Δ26Mgcalcite-seawater = −2.6 ± 0.3‰ that appears to be weakly related to temperature. With one exception (Vaceletia spp.), aragonitic corals and sclerosponges also display uniform Mg isotope fractionations relative to seawater with Δ26Mgbiogenic aragonite-seawater = −0.9 ± 0.2.Magnesium isotopes in high-Mg calcites from red algae, echinoids and perhaps some porcelaneous foraminifera as well as in all low-Mg calcites (perforate foraminifera, coccoliths and brachiopods) display significant biological influences. For planktonic foraminifera, the Mg isotope data is consistent with the fixation of Mg by organic material under equilibrium conditions, but appears to be inconsistent with Mg removal from vacuoles. Our preferred model, however, suggests that planktonic foraminifera synthesize biomolecules that increase the energetic barrier for Mg incorporation. In this model, the need to remove large quantities of Mg from vacuole solutions is avoided. For the high-Mg calcites from echinoids, the precipitation of amorphous calcium carbonate may be responsible for their weaker Mg isotope fractionation.Disregarding superimposed biological effects, it appears that cation light isotope enrichments in CaCO3 principally result from a chemical kinetic isotope effect, related to the incorporation of cations at kink sites. In this model, the systematics of cation isotope fractionations in CaCO3 relate to the activation energy required for cation incorporation, which probably reflects the dehydration of the cation and the crystal surface and bond formation at the incorporation site. This kinetic incorporation model predicts (i) no intrinsic dependence on growth rate, unless significant back reaction upon slow growth reduces the isotope fractionation towards that characteristic for equilibrium isotope partitioning (this may be observed for Ca isotopes in calcites), (ii) a small decrease of isotope fractionation with increasing temperature that may be amplified if higher temperatures promote back reaction and (iii) a sensitivity to changes in the activation barrier caused by additives such as anions or biomolecules or by the initial formation of amorphous CaCO3. 相似文献
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Sascha Fl?gel Geoffrey Parkin Dave Pollard Wolf-Christian Dullo Thomas Wagner 《Climate Dynamics》2011,37(7-8):1565-1573
Freshwater discharge is one main element of the hydrological cycle that physically and biogeochemically connects the atmosphere, land surface, and ocean and directly responds to changes in pCO2. Nevertheless, while the effect of near-future global warming on total river runoff has been intensively studied, little attention has been given to longer-term impacts and thresholds of increasing pCO2 on changes in the partitioning of surface and subsurface flow paths across broad climate zones. These flow paths and their regional responses have a significant role for vegetation, soils, and nutrient leaching and transport. We present climate simulations for modern, near-future (850?ppm), far-future (1880?ppm), and past Late Cretaceous (1880?ppm) pCO2 levels. The results show large zonal mean differences and the displacement of flows from the surface to the subsurface depending on the respective pCO2 level. At modern levels the ratio of deeper subsurface to near-surface flows for tropical and high northern latitudes is 1:4.0 and 1:0.5, respectively, reflecting the contrast between permeable tropical soils and the areas of frozen ground in high latitudes. There is a trend toward increased total flow in both climate zones at 850?ppm, modeled to be increases in the total flow of 34 and 51%, respectively, with both zones also showing modest increases in the proportion of subsurface flow. Beyond 850?ppm the simulations show a distinct divergence of hydrological trends between mid- to high northern latitudes and tropical zones. While total wetting reverses in the tropics beyond 850?ppm due to reduced precipitation, with average zonal total runoff decreasing by 46% compared to the 850?ppm simulation, the high northern latitude zone becomes slightly wetter with the average zonal total runoff increasing by a further 3%. The ratio of subsurface to surface flows in the tropics remains at a level similar to the present day, but in the high northern latitude zone the ratio increases significantly to 1:1.6 due to the loss of frozen ground. The results for the high pCO2 simulations with the same uniform soil and vegetation cover as the Cretaceous are comparable to the results for the Cretaceous simulation, with higher fractions of subsurface flow of 1:5.4 and 1:5.6, respectively for the tropics, and 1:2.2 and 1:1.6, respectively for the high northern latitudes. We suggest that these fundamental similarities between our far future and Late Cretaceous models provide a framework of possible analogous consequences for (far-) future climate change, within which the integrated human impact over the next centuries could be assessed. The results from this modeling study are consistent with climate information from the sedimentary record which highlights the crucial role of terrestrial-marine interactions during past climate change. This study points to profound consequences for soil biogeochemical cycling, with different latitudinal expressions, passing of climate thresholds at elevated pCO2 levels, and enhanced export of nutrients to the ocean at higher pCO2. 相似文献
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Wolf-Christian Dullo 《International Journal of Earth Sciences》2009,98(5):1095-1095
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International Journal of Earth Sciences - Karl Andrée began studying questions of sedimentology and oceanography in 1908 when working as an assistant at the University of Marburg and he... 相似文献