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1.
230Th, 232Th and 234Th were analyzed in sinking particles collected by moored and drifting sediment traps in the NE Atlantic Ocean (POMME experiment) in order to constrain the phase(s) carrying Th isotopes in the water column. It reveals a contrasted behaviour between 234Th and 230Th. 234Th is correlated to the particulate organic carbon suggesting that it is primarily scavenged by organic compounds in the surface waters. 230Thxs is correlated with Mn, Ba and the lithogenic fraction that are enriched in small suspended particles and incorporated in the sinking particulate flux throughout the water column. The lack of correlation between 230Thxs and CaCO3 or biogenic silica (bSi) indicates that CaCO3 and bSi are not responsible for 230Th scavenging in the deep waters of this oceanic region. 230Th is generally correlated with the lithogenic content of the trapped material but this correlation disappears in winter during strong atmospheric dust inputs suggesting that lithogenic matter is not directly responsible for 230Th scavenging in the deep waters or that sufficient time is required to achieve particle–solution equilibration. MnO2 could be the prevalent 230Thxs-bearing phase. The narrow range of Kd_MnO2Th obtained for very contrasted oceanic environments supports a global control of 230Thxs scavenging by MnO2 and raises the possibility that the 230Th–231Pa fractionation is controlled by the amount of colloidal MnO2 in seawater.  相似文献   

2.
We investigated the influence of the composition of the vertical particle flux on the removal of particle reactive natural radionuclides (230Th and 231Pa) from the water column to the sediments. Radionuclide concentrations determined in sediment traps moored in the western, central and eastern Arabian Sea were related to the major components (carbonate, particulate organic matter (POC), opal, lithogenic material) of the particle flux. These data were combined with sediment trap data previously published from the Southern Ocean, Equatorial Pacific and North Atlantic [Z. Chase, R.F. Anderson, M.Q. Fleisher, P.W. Kubik, The influence of particle composition and particle flux on scavenging of Th, Pa and Be in the ocean, Earth Planet. Sci. Lett. 204 (2002) 215–229; J.C. Scholten, F. Fietzke, S. Vogler, M. Rutgers van der Loeff, A. Mangini, W. Koeve, J. Waniek, P. Stoffers, A. Antia, J. Kuss, Trapping efficiencies of sediment traps from the deep eastern North Atlantic: The 230Th calibration, Deep Sea Research II 48 (2001) 2383–2408]. The correlations observed between the particle-dissolved distribution coefficients (Kd) of 230Th and 231Pa and the concentrations of the particle types depend on the sediment trap data set used. This result suggests that scavenging affinities of the nuclides differ between oceanic regions. Several factors (Kd values, reactive surface areas of particles, inter-correlations in closed data set) can, however, influence the observed relationships and thus hamper the interpretation of these correlation coefficients as a measure of relative scavenging affinities of the nuclides to the particle types investigated. The mean fractionation factor (F(Pa/Th)=Kd(Pa)/Kd(Th)) from the Equatorial Pacific (F=0.11±0.03) is similar to that from the North Atlantic (F(Pa/Th)=0.077±0.026), and both are lower than the factors from the Arabian Sea (F(Pa/Th)=0.35±0.12) and from the Southern Ocean (F(Pa/Th)=0.87±0.4). For opal concentrations exceeding 60%, an increase in the fractionation factors is observed causing a higher mean fractionation factor for the Southern Ocean trap data set. For the other areas investigated, differences in the mean fractionation factors cannot be related to the particles types considered. In the Arabian Sea, seasonally variable 231Paex/230Thex ratios observed in the sediment traps as well as differences of the ratios between recently deposited phytodetritus (fluff) and normal surface sediments indicate seasonal changes in scavenging processes which the generally accepted reversible scavenging models do not envisage. We assume that variable sinking rates of particles, and/or particles not considered in this study (e.g. colloids, manganese oxides, transparent exopolymer particles) may play an important but as yet unexplored role in deep-water scavenging processes.  相似文献   

3.
Two diagenetic manganese nodules from the Peru Basin were investigated by thermal ionization mass spectrometry and high resolution alpha spectrometry for uranium and thorium. The TIMS concentrations for nodule 62KD (63KG) vary as follows: 0.12–1.01 ppb (0.06–0.59) 230Th, 0.51–1.98 ppm (0.43–1.40) 232Th, 0.13–0.80 ppb (0.09–0.49) 234U, and 1.95–13.47 ppm (1.66–8.24) 238U. Both nodules have average growth rates of 110 mm per million years. However, from the variations of excess 230Th with depth we estimate partial accumulation rates which range from 50 to 400 mm per million years. The δ234U dating method cannot be applied due to remobilization of U from the sediment and subsequent incorporation into the nodules' crystal lattice, reflected by decay corrected δ234U values far above the ocean water value. Sections of fast nodule growth are related to those layers having high Mn/Fe ratios (up to 200) and higher densities. As a possible explanation we develop a scenario that describes similar glacial/interglacial trends in both nodules as a record of regional changes of sediment and/or deep water chemistry.  相似文献   

4.
238U, 232Th, 230Th and 226Ra abundances have been measured in six samples of recent Hawaiian basalt by high precision mass spectrometry, in an attempt to compare the melting process in plumes and at spreading ridges. The data reveal a very small range in (230Th/238U) activity ratio up to a maximum value of 1.02 ± 0.01, and (226Ra/230Th) activity ratios which lie between 1.10 ± 0.015 and 1.19 ± 0.02. UTh and RaTh abundances are linearly correlated demonstrating that the disequilibria predate crystallisation and differentiation. Using recently published estimates for the bulk partition coefficients of U and Th, the results are consistent with melting rates > 10−3 kg m−3 a−1 at porosities < 10−3 for dynamic fractional melting in the garnet stability field.  相似文献   

5.
We have developed techniques to determine238U,234U and232Th concentrations in seawater by isotope dilution mass spectrometry. U measurements are made using a233U236U double spike to correct for instrumental fractionation. Measurements on uranium standards demonstrate that234U/238U ratios can be measured accurately and reproducibly.234U/238U can be measured routinely to ± 5‰ (2σ) for a sample of 5 × 109 atoms of234U (3 × 10−8 g of total U, 10 ml of seawater). Data acquisition time is 1 hour. The small sample size, high precision and short data acquisition time are superior to-counting techniques.238U is measured to ± 2‰ (2σ) for a sample of 8 × 1012 atoms of238U ( 3 × 10−9 g of U, 1 ml of seawater).232Th is measured to ± 20‰ with 3 × 1011232Th atoms (10−10 g232Th, 1 1 of seawater). This small sample size will greatly facilitate investigation of the232Th concentration in the oceans. Using these techniques, we have measured238U,234U and232Th in vertical profiles of unfiltered, acidified seawater from the Atlantic and238U and234U in vertical profiles from the Pacific. Determinations of234U/238U at depths ranging from 0 to 4900 m in the Atlantic (7°44′N, 40°43′W) and the Pacific (14°41′N, 160°01′W) Oceans are the same within experimental error (± 5‰,2σ). The average of these234U/238U measurements is 144 ± 2‰ (2σ) higher than the equilibrium ratio of 5.472 × 10−5. U concentrations, normalized to 35‰ salinity, range from 3.162 to 3.281 ng/g, a range of 3.8%. The average concentration of the Pacific samples (31°4′N, 159°1′W) is 1% higher than that of the Atlantic (7°44′N, 40°43′W and 31°49′N, 64°6′W).232Th concentrations from an Atlantic profile range from 0.092 to 0.145 pg/g. The observed constancy of the234U/238U ratio is consistent with the predicted range of234U/238U using a simple two-☐ model and the residence time of deep water in the ocean determined from14C. The variation in salinity-normalized U concentrations suggests that U may be much more reactive in the marine environment than previously thought.  相似文献   

6.
汪建军  许才军 《地球物理学报》2017,60(11):4398-4420
2017年8月8日21时19分在我国四川省北部阿坝州九寨沟县发生了MW6.5左旋走滑型地震.该地震发生在青藏高原巴颜喀拉块体东北缘,东昆仑断裂南东段的塔藏断裂、岷江断裂和虎牙断裂的交汇地带.包括此次地震,近年来在巴颜喀拉块体周缘已发生了九次6级以上强震,表明巴颜喀拉块体周缘主要活动断裂上的应力水平仍处于不断调整之中.本文采用库仑应力模型研究2017年MW6.5九寨沟地震激发的库仑应力变化、该地震与周边地震的应力触发关系以及强震对周边主要活动断裂的应力扰动.强震序列包括周边区域1536-1975年M≥6历史强震和1976-2017年的MW≥ 6 gCMT地震目录中的强震,共计32个.研究结果表明：（1）2017年MW6.5九寨沟地震激发的同震库仑应力变化仅在局部范围内超过0.1×105Pa,且75%的余震（~12.7天）受到该地震明显的同震应力触发作用,而其余25%的余震落在应力影区,采用最优破裂面可以进一步提高同震库仑应力变化与余震分布的空间相关性;（2）2008年MW7.9汶川地震对2017年MW6.5九寨沟地震的发生有一定的促进作用,在后者震源处激发的同震库仑应力变化为（0.026~0.263）×105Pa,震后黏弹性库仑应力变化为（0.010~0.032）×105Pa.该库仑应力的变化范围取决于汶川地震源断层参数和九寨沟地震接收断层参数.2013年MW6.6芦山地震对九寨沟地震的发生几乎没有影响（< 0.001×105Pa）;（3）1654年M8.0甘肃天水南地震对九寨沟地震的发生有明显的促进作用,在九寨沟地震震源处激发的同震库仑应力变化为（0.410~1.266）×105Pa,震后库仑应力变化为（0.147~0.490）×105Pa.1879年M8.0甘肃武都地震可能有比1654年M8.0甘肃天水南地震更强的应力触发作用,但也有可能对九寨沟地震的发生起到抑制作用.在选取的8个九寨沟地震接收断层面上,其中6个接收断层面上该地震所激发的同震库仑应力变化为（0.913~2.364）×105Pa,2个接收断层面上该地震所激发的同震库仑应力变化为（－1.326~－0.454 ）×105Pa;在4个接收断层面上震后库仑应力变化为（0.094~1.072）×105Pa,在另外4个接收断层面上震后库仑应力变化为（－1.593~－0.106）×105Pa.1933年四川叠溪地震对九寨沟地震的发生影响较弱,其所激发的同震库仑应力变化为（0.015~0.080）×105Pa,震后库仑应力变化为（－0.029~0.025）×105Pa;（4）九寨沟地震仅在其附近的岷江断裂北段、塔藏断裂和虎牙断裂南段造成较明显的同震库仑应力变化,其分别为0.09×105Pa、（0.14~2.03）×105Pa和0.25×105Pa.而进一步顾及其余31个强震的库仑应力作用则发现,同震库仑应力增加非常显著的主要活动断裂分段为：岷江断裂北段南侧和岷江断裂南段的库仑应力变化分别升高5.6×105Pa和9.8×105Pa.鲜水河断裂北段南侧库仑应力升高23.0×105Pa,鲜水河断裂南段道孚-康定段的北侧库仑应力升高9.0×105Pa,而最南端库仑应力升高3.0×105Pa;龙门山断裂带中段的北侧库仑应力变化为（6.1~7.4）×105Pa,中段库仑应力增加（2.1~11.5）×105Pa;西秦岭北缘断裂东段库仑应力变化为4.4×105Pa;龙日坝断裂北段最北侧的库仑应力变化为2.0×105Pa;小金河断裂北段库仑应力变化为1.7×105Pa;安宁河断裂北段库仑应力变化为1.6×105Pa;（5）由于下地壳和上地幔的黏弹性松弛作用,所有强震在九寨沟地震震后20年造成的黏弹性库仑应力变化在鲜水河断裂、龙门山断裂中段、塔藏断裂以及秦岭南缘断裂西段比较显著,其分别为：（1.0~3.0）×105Pa、2.8×105Pa、（2.3~2.7）×105Pa和0.9×105Pa.但总体上黏弹性库仑应力变化没有改变各断裂上的同震库仑应力变化空间分布.总的库仑应力变化在鲜水河断裂北段南侧和南段的道孚至康定段北侧、龙门山断裂中段北侧、岷江断裂南段和北段南侧、虎牙断裂、塔藏断裂以及西秦岭北缘东段很显著（均超过4×105Pa）.由于库仑应力明显升高可能预示着地震潜在危险性增强,因此这些断裂分段可能将来需要重点加以关注.  相似文献   

7.
Two sediment cores (pilot gravity and piston) were obtained from the bottom of the Izu-Ogasawara Trench at 9750 m and analyzed for various elements and radioisotopes. The results showed a history of complex and frequent turbidite deposition: In the gravity core, eight layers rich in manganese were observed, of which five are enriched in Cu and Co as well. The other three are also enriched in Mo but no other heavy metals, suggesting the presence of at least two mechanisms of formation. Trapping of iron manganese micronodules can account for the enrichment of Mn, Cu and Co. The other three layers rich in Mn and Mo appear to be formed by a post-depositional diagenetic process of Mn mobilization and redeposition in the sediment column. A strong correlation between 226Ra and Cu in the gravity core suggests that the 226Ra was also carried into the bottom of the trench in turbidites in association with Mn micronodules. Little excess of 210Pb over 226Ra was found at the top but the excess was significant at mid-depths from 30 to 70 cm, indicating that those sediments were deposited within the last 200 y.

In the piston core there is a sharp discontinuity of chemical and radiochemical composition around a depth of 250 cm. Below that depth the sediments appear to be dominated by materials derived from terrestrial sources, as compared with those in the upper layer which are of contemporary marine origin. 226Ra is deficient relative to 230Th throughout the sediment column down to 6 m. This finding is consistent with the finding that the average rate of sediment accumulation is 1–2 orders of magnitude faster than that in the western North Pacific abyssal plain, suggesting the convergence of materials into the bottom of the trench.  相似文献   


8.
A chronology based on optically stimulated luminescence (OSL) dating is presented for the late- and post-glacial evolution of the southern Baltic Sea (15 ka to present). During this period, large water level and salinity changes occurred in the Baltic Basin due to opening and closing of connections to the North Atlantic. Previous attempts to establish a chronology for these palaeoenvironmental changes have mainly been conducted in coastal settings where organic material for 14C dating is abundant. Many of these records are, however, discontinuous due to the large water level fluctuations. In contrast, in the relatively deep water of the Arkona Basin, the sediment record is expected to be more or less continuous. The single aliquot regenerative dose (SAR) procedure was used to date 32 samples of fine quartz sand from a 10.86 m long sediment core from the centre of this basin (45 m water depth). Tests of luminescence characteristics confirmed the suitability of the material for OSL dating and the ages agree well with the available AMS 14C ages on shells. The Baltic Ice Lake drainage to the North Atlantic appears to occur 11.6 ka, agreeing with other published evidence. However, we suggest that the main marine Littorina transgression appears in the Arkona Basin at about 6.5 ka, rather than at 8.5 ka, as previously thought.  相似文献   

9.
In a series of observing system simulations, we test whether the Atlantic meridional overturning circulation (AMOC) can be observed based on the existing Line W deep western boundary array. We simulate a Line W array, which is extended to the surface and to the east to cover the basin to the Bermuda Rise. In the analyzed ocean circulation model ORCA025, such an extended Line W array captures the main characteristics of the western boundary current. Potential trans-basin observing systems for the AMOC are tested by combining the extended Line W array with a mid-ocean transport estimate obtained from thermal wind “measurements” and Ekman transport to the total AMOC (similarly to Hirschi et al., Geophys Res Lett 30(7):1413, 2003). First, we close Line W zonally supplementing the western boundary array with several “moorings” in the basin (Line W-32°N). Second, we supplement the western boundary array with a combination of observations at Bermuda and the eastern part of the RAPID array at 26°N (Line W-B-RAPID). Both, a small number of density profiles across the basin and also only sampling the eastern and western boundary, capture the variability of the AMOC at Line W-32°N and Line W-B-RAPID. In the analyzed model, the AMOC variability at both Line W-32°N and Line W-B-RAPID is dominated by the western boundary current variability. Away from the western boundary, the mid-ocean transport (east of Bermuda) shows no significant relation between the two Line W-based sections and 26°N. Hence, a Line W-based AMOC estimate could yield an estimate of the meridional transport that is independent of the 26°N RAPID estimate. The model-based observing system simulations presented here provide support for the use of Line W as a cornerstone for a trans-basin AMOC observing system.  相似文献   

10.
Mass spectrometric techniques have recently yielded significant increases in precision and sensitivity over previous methods for measuring230Th abundance in corals. To assess the accuracy of230Th ages, three corals from Vanuatu, whose ages were known from counting annual growth bands, were analyzed. For each sample, the date of growth determined by230Th analysis (A.D.1969 ± 3, 1932 ± 5,and1806 ± 5; 2σ uncertainties based on analytical error) was indistinguishable from the date determined by counting bands (A.D. 1971–1973, 1935–1937, and 1804–1810), indicating that the230Th dates are accurate.230Th dates were also determined for two adjacent emerged heads from Santo Is., Vanuatu, which were thought to have died when they were raised above sea level during coseismic uplift. The dates (A.D.1864 ± 4, 1866 ± 4) were the same, indicating that the heads died at the same time and consistent with the idea that they were killed by coseismic emergence around A.D. 1865. The difference between this date and the date of the only major historically documented earthquake that caused uplift (A.D. 1973,Ms = 7.5), suggests a seismic recurrence interval of108 ± 4y for Santo. Analogous emerged corals from Malekula Is., Vanuatu yielded230Th dates that were similar to each other (A.D.1729 ± 3, 1718 ± 5) and are inferred to have died during coseismic emergence around A.D. 1729. In conjunction with the date of the only large historically documented earthquake that caused uplift (A.D. 1965,Ms = 7.5), the recurrence interval for Malekula is236 ± 3y. If similar emerged corals can be found, this appraoch may be extended back in time and to other localities because it appears that such features can now be dated both accurately and precisely.  相似文献   

11.
Changes in oceanic radiocarbon (14C) reservoir ages through the deglaciation and Holocene can provide important information on ocean circulation as Earth's climate warmed. Here, we present reservoir ages for the western tropical Pacific that span the mid-Holocene transition from less to more frequent El Niño events. Reservoir ages were calculated using paired U–Th and conventional 14C dating of eight individual fossil coral samples from Koil and Muschu Islands, northern coastal Papua New Guinea (PNG). AMS 14C and MC-ICPMS U–Th dating of additional samples from six of the fossil corals were used to confirm the TIMS U–Th and conventional 14C ages. The combined results show average reservoir ages of 185±30 14C yr (n=4) for 7220–5850 yr BP compared to 420 14C yr for a modern coral from Muschu Island. From 5850 to 5420 yr BP reservoir ages increase to modern values. The relatively young reservoir ages from 7220 to 5850 yr BP are best explained by greater influx of well-equilibrated sub-tropical water from the southern branch of the South Equatorial Current (SEC). This is consistent with strengthening trade winds (facilitating air–sea exchange) and a more northerly position of the Intertropical Convergence Zone thought to have occurred at this time. The transition to more modern-like reservoir ages from 5850 to 5420 yr BP suggests modern oceanic circulation patterns were established during this interval. The onset of modern El Niño activity around this time would have served to enhance the intrusion of 14C-depleted equatorial waters via the south equatorial branch of the SEC. Overall, the changes in reservoir age presented here for the western tropical Pacific suggest that Holocene changes in the El Niño–Southern Oscillation state of the tropical Pacific resulted in reorganisation of oceanic circulation in this region.  相似文献   

12.
Water sampling during the 1993 IV Russian–US Joint Expedition to the Bering and Chukchi Seas (BERPAC) indicates that Pacific Ocean burdens of the long-lived radionuclide 129I are relatively low in the Pacific-influenced Arctic, particularly compared to high latitude waters influenced by the North Atlantic. These low concentrations occur despite the presence of potential submerged anthropogenic sources in the East Sea (Sea of Japan), and in the northwest Pacific Ocean, east of the Kamchatka Peninsula. The concentration of 129I entering the Arctic Ocean through Bering Strait, 0.7×108 atoms kg−1, is only slightly higher than observed in deep Pacific waters. Similar concentrations (0.44–0.76×108 atoms kg−1) measured in Long Strait indicate no significant transfer of 129I eastward into the Chukchi Sea in the Siberian Coastal Current from the Siberian marginal seas to the west. However, the concentrations reported here are more than an order of magnitude higher than the Bering Strait input concentration estimated (1.0×106 atoms kg−1) from bomb fallout mass balances, which supports other existing evidence for a significant atmospheric deposition term for this radionuclide in surface ocean waters. Near-bottom water samples collected in productive waters of the Bering and Chukchi Seas also suggest that sediment regeneration may locally elevate 129I concentrations, and impact its utility as a water mass tracer. As part of this study, two deep 129I profiles were also measured in the East Sea in 1993–1994. The near-surface concentration of 129I ranged from 0.12 to 0.31×108 atoms kg−1. The 129I concentration showed a steady decrease with depth, although because of active deep water ventilation, the entire 3000 m water column exceeded natural concentrations of the radionuclide. Atom ratios of 129I/137Cs in the East Sea also suggest an excess of 129I above bomb fallout estimates, also possibly resulting from atmospheric deposition ultimately originating from nuclear facilities.  相似文献   

13.
The Late Neogene witnessed various major paleoceanographic changes that culminated in intense Northern Hemisphere Glaciation (NHG). The cause and effects of these changes are still debated. We use a multiproxy approach to determine the relative timing of the closure of the Panama gateway, changes in Atlantic circulation, global cooling and ice sheet growth. Benthic foraminiferal Mg/Ca records from a Pacific and an Atlantic Site have been produced and are interpreted in terms of bottom water temperatures. These Mg-temperature records are combined with published benthic δ13C, δ18O and erosion records to reconstruct the flow of proto-North Atlantic Deep Water (proto-NADW) over the past 12 Ma. The results suggest that between 12.5 and 10.5 Ma, and again between about 8.5 and 6 Ma, a nutrient-depleted water mass that was colder (by 1–2°C) and fresher than the intervening deep water mass filled the Atlantic basin. This proto-NADW became warmer (by 1°C) and saltier between 6 and 5 Ma, coincident with the restriction of surface water flow through the Central American Seaway. The Mg-temperature records define a subsequent global cooling trend of 3.5°C between 5 Ma and today. Early NHG in the late Miocene was perhaps related to the formation of the relatively cold, fresh proto-NADW. The formation of the warmer and saltier proto-NADW in the early Pliocene may have initially limited Northern Hemisphere ice growth. However, the increased moisture released at high northern latitudes associated with formation of ‘warm’ proto-NADW, coupled with the global temperature decrease of deep (and hence polar surface) waters, likely helped initiate the intense NHG of the Plio–Pleistocene.  相似文献   

14.
Lithologic data compiled from Deep Sea Drilling Project and Ocean Drilling Program sites, when combined with orthogonal convergence rates at convergent plate boundaries, permit quantification of the mass flux of sediment into subduction zones. We have made such calculations for each major sediment component — terrigenous grains, calcium carbonate, opal, and water — for twelve trench systems. Results show that 1.4 × 1015 g/yr of sediment and 0.9 × 1015 g/yr of water enter the trenches in the oceanic sedimentary layer. Most of the entering sediment, 1.1 × 1015 g/yr, is terrigenous; the remainder is more carbonate than opal. For most of geologic time an order of magnitude more sediment enters the ocean than leaves it via subduction. The global sedimentary cycle need be in balance only over an entire Wilson cycle. Comparison of sediment fluxes into trenches with the magnitude of large earthquakes and with the composition of bulk volcanic rock shows no correlation.  相似文献   

15.
Measurements of 231Pa, 230Th and 232Th concentrations have been made on five water-column profiles along the western margin of the Madagascar and Mascarene Basins in the southern Indian Ocean. These measurements help to fill a significant gap in the global coverage of water-column 232Th, 230Th and 231Pa data. 232Th concentrations vary, but generally increase with depth, suggesting higher particle loading in deeper waters, and the presence of a significant dissolved fraction of 232Th. 230Th concentrations increase with depth, and profiles are similar to the average of existing data from other regions. 231Pa concentrations, on the other hand, show significant depth structure, apparently reflecting the various water masses sampled at this location. The modified remnants of North Atlantic Deep Water are found at a depth of ≈ 2000 m and exhibit elevated 231Pa concentrations exported from the South Atlantic. Antarctic Intermediate and Bottom Waters have lower 231Pa, probably due to scavenging onto opal particles during transit from the Southern Ocean. The differences between water masses raises a question: which water mass is important in controlling the 231Pa/230Th ratio in underlying sediments? A simple one-dimensional model is used to demonstrate that the 230Th and 231Pa exported to sea-floor sediments last equilibrates with waters close to the seafloor (within ≈ 1000 m), rather than averaging the whole water column. These findings suggest that 231Paxs/230Thxs in sediments provides information primarily about deep-water masses. In this region, sedimentary records will therefore provide information about the past flow of Antarctic Bottom Water into the Indian Ocean. Interpretation of data from other regions, such as the North Atlantic where this proxy has most successfully been applied, requires careful consideration of regional oceanography and knowledge of the composition of the water masses being investigated.  相似文献   

16.
A sediment core from the western tropical Atlantic covering the last 21,000 yr has been analysed for centennial scale reconstruction of sea surface temperature (SST) and ice volume-corrected oxygen isotopic composition of sea water (δ18Oivc-sw) using Mg / Ca and δ18O of the shallow dwelling planktonic foraminifer Globigerinoides ruber (white). At a period between 15.5 and 17.5 kyr BP, the Mg / Ca SST and δ18Oivc-sw, a proxy for sea surface salinity (SSS), reveals a warming of around 2.5 °C along with an increase in salinity. A second period of pronounced warming and SSS increase occurred between 11.6 and 13.5 kyr BP. Within age model uncertainties, both warming intervals were synchronous with air temperature increase over Antarctica and ice retreat in the southern South Atlantic and terminated with abrupt centennial scale SSS decrease and slight SST cooling in conjunction with interglacial reactivation of the meridional overturning circulation (MOC). We suggest that during these warm intervals, production of saline and warm water of the North Brazil Current resulted in pronounced heat and salt accumulation, and was associated with warming in the southern Atlantic, southward displacement of the intertropical convergence zone and weakened MOC. At the termination of the Younger Dryas and Heinrich event 1, intensification of cross-equatorial heat and salt transport caused centennial scale cooling and freshening of the western tropical Atlantic surface water. This study shows that the western tropical Atlantic served as a heat and salt reservoir during deglaciation. The sudden release of accumulated heat and salt at the end of Younger Drays and Heinrich event 1 may have contributed to the rapid reinvigoration of the Atlantic MOC.  相似文献   

17.
Volcanic rocks from subduction zones are widely believed to originate by partial melting of mantle lherzolite modified by the addition of a fluid or melt extracted from the down-going slab. U-series disequilibrium in such magmas is commonly attributed to this particular melting process. A detailed study of U-series isotopes in the 650 y. B.P. eruptive sequence of Mt. Pelée (Martinique) shows that plinian products are in radioactive equilibrium, whereas dome-forming products of the same eruption are characterized by 238U-230Th disequilibrium. The same features apply to other plinian and dome-forming products of this volcano and systematically correspond to different eruptive styles. We attribute these characteristics to variable superficial interaction of magmas with the hydrothermal system during the final stages of eruption rather than to deep magma genesis processes. This conclusion might be generally applicable to arc magmas.  相似文献   

18.
Deep circulation driven by strong vertical mixing in the Timor Basin   总被引：1，自引：1，他引：0  
The importance of deep mixing in driving the deep part of the overturning circulation has been a long debated question at the global scale. Our observations provide an illustration of this process at the Timor Basin scale of ～1000 km. Long-term averaged moored velocity data at the Timor western sill suggest that a deep circulation is present in the Timor Basin. An inflow transport of ～0.15 Sv is observed between 1600 m and the bottom at 1890 m. Since the basin is closed on its eastern side below 1250 m depth, a return flow must be generated above 1600 m with a ～0.15 Sv outflow. The vertical turbulent diffusivity is inferred from a heat and transport balance at the basin scale and from Thorpe scale analysis. Basin averaged vertical diffusivity is as large as 1 × 10?3 m2 s?1. Observations are compared with regional low-resolution numerical simulations, and the deep observed circulation is only recovered when a strong vertical diffusivity resulting from the parameterization of internal tidal mixing is considered. Furthermore, the deep vertical mixing appears to be strongly dependent on the choice of the internal tide mixing parameterization and also on the prescribed value of the mixing efficiency.  相似文献   

19.
Geochemical variations in mid-ocean ridge basalts have been attributed to differing proportions of compositionally distinct mantle components in their sources, some of which may be recycled crust. Oxygen isotopes are strongly fractionated by near-surface interactions of rocks with the hydrosphere, and thus provide a tracer of near-surface materials that have been recycled into the mantle. We present here oxygen isotope analyses of basaltic glasses from the mid-Atlantic ridge south of and across the Azores platform. Variations in δ18O in these samples are subtle (range of 0.47‰) and may partly reflect shallow fractional crystallization; we present a method to correct for these effects. Relatively high fractionation-corrected δ18O in these samples is associated with geochemical indices of enrichment, including high La/Sm, Ce/Pb, and 87Sr/86Sr and low 143Nd/144Nd. Our results suggest two first-order conclusions about these enriched materials: (1) they are derived (directly or indirectly) from recycled upper oceanic crustal rocks and/or sediments; and (2) these materials are present in the north Atlantic MORB sources in abundances of less than 10% (average 2–5%). Modeling of variations of δ18O with other geochemical variables further indicates that the enriched component is not derived from incorporation of sediment or bulk altered oceanic crust, from metasomatism of the mantle by hydrous or carbonate-rich fluids, or from partial melting of subducted sediment. Instead, the data appear to require a model in which the enriched component is depleted mantle that has been metasomatized by small-degree partial melts of subducted, dehydrated, altered oceanic crust. The age of this partial melting is broadly constrained to 250 Ma. Reconstructed plate motions suggest that the enriched component in the north Atlantic mantle may have originated by subduction along the western margin of Pangea.  相似文献   

20.
Marine carbonate skeletons like deep-sea corals are frequently coated with iron and manganese oxides/hydroxides which adsorb additional thorium and uranium out of the sea water. A new cleaning procedure has been developed to reduce this contamination. In this further cleaning step a solution of Na2EDTA (Na2H2TB) and ascorbic acid is used which composition is optimised especially for samples of 20 mg of weight. It was first tested on aliquots of a reef-building coral which had been artificially contaminated with powdered ferromanganese nodule. Applied on heavily contaminated deep-sea corals (scleractinia), it reduced excess 230Th by another order of magnitude in addition to usual cleaning procedures. The measurement of at least three fractions of different contamination, together with an additional standard correction for contaminated carbonates results in Th/U-ages corrected for the authigenic component. A good agreement between Th/U- and 14C-ages can be achieved even for extremely coated corals.  相似文献   

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