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21.
We present an approach for tracing the fate of anthropogenic CO2 , compiling a large data set of stable organic carbon isotope ratios from surface sediments, plankton, and sinking matter in the Atlantic Ocean. The δ13 C values of sinking matter are generally lower by 0.5–4.6‰ compared to the surface sediments. This difference increases with increasing latitude, which is explained by a stronger modern increase in surface water [CO2 (aq)] in the Southern Ocean relative to the Tropical/Subtropical Ocean. Preindustrial dissolved CO2 concentrations in Atlantic surface waters, estimated from the δ13 Corg of surface sediments, are compared to recently measured surface water [CO2 (aq)] values taken from literature. We obtain only a slight increase in [CO2 (aq)] at lower latitudes but a significant change of about 7 ± 2 μ m in high latitudinal surface waters which we attribute to anthropogenic perturbation. Our results suggest that CO2 released by human activities has been stored in Southern Ocean surface waters. 相似文献
22.
Eastern boundary upwelling is an important aspect of the modern ocean, despite the relatively small area involved. Consequences of increased upwelling during the Neogene, as a result of intensifying trade winds, include increased heat transport by the ocean, increased mixing, increased fractionation of phosphate from silicate, and opportunities for evolution of new species. Coastal upwelling has been studied since the first decade of the 20th century. The processes involved are complex, and differ depending on geographic setting. Off Namibia, upwelling history has been studied by a number of drilling expeditions; the last of these was ODP Leg 175, which occupied seven sites on the continental slope between Walvis Ridge and Cape Town, during September and October of 1997. Productivity proxies all along southwestern Africa suggest the presence of increased upwelling and organic matter supply to the sea floor during glacial periods. At the same time, there is a decrease in the supply of diatoms and other siliceous plankton remains. This is the Walvis Opal Paradox, established through the contrasting results of studies by Diester-Haass (1985) and Oberhänsli (1991). We propose that the Walvis Opal Paradox is fundamental to the understanding of glacial–interglacial productivity fluctuations on a global scale. Furthermore, a central feature of the history of late Neogene upwelling off Namibia is the Matuyama Opal Maximum, centered between Gauss and Olduvai magnetic chrons [Wefer et al., Proc. ODP 175 (1998)]. It is due to the fact that diatom supply first increases (during the Gauss) when the planet cools and then decreases again during additional cooling, on entering the Quaternary. On a 400 000-year scale, peak productions are coincident with (or slightly lag) maximum seasonal contrast potential in the high-latitude insolation curve. We suggest that this is further evidence that the nutrient content of thermocline waters was diminished during glacial periods. The reasons why this should be so remain to be discovered. 相似文献
23.
24.
Link between the North and South Atlantic during the Heinrich events of the last glacial period 总被引:2,自引:0,他引:2
L. Vidal R.R. Schneider O. Marchal T. Bickert T.F. Stocker G. Wefer 《Climate Dynamics》1999,15(12):909-919
High resolution benthic oxygen isotope records combined with radiocarbon datings, from cores retrieved in the North, Equatorial,
and South Atlantic are used to establish a reliable cronostratigraphy for the last 60 ky. This common temporal framework enables
us to study the timing of the sub-Milankovitch climate variability in the entire surface Atlantic during this period, as reflected
in planktonic oxygen isotope records. Variations in sea surface temperatures in the Equatorial and South Atlantic reveal two
warm periods during the mid-stage 3 which are correlated to the warming observed in the North Atlantic after Heinrich events
(HL) 5 and 4. However, the records show that the warming started about 1500 y earlier in the South Atlantic. A zonally averaged
ocean circulation model simulates a similar north-south thermal antiphasing between the latitudes of our coring sites, when
pertubated by a freshwater flux anomaly. We infer that the observed phase relationship between the northern and the southern
Atlantic is related to periods of reduced NADW production in the North Atlantic, such as during HL5 and HL4.
Received: 24 November 1998 / Accepted: 16 May 1999 相似文献
25.
H. Kuhnert J. Pätzold K. -H. Wyrwoll G. Wefer 《International Journal of Earth Sciences》2000,88(4):725-732
116-year record of coral skeletal δ18O is presented from a colony of Porites lutea from Ningaloo Reef, western Australia. Interannual variability of sea-surface temperatures (SST) inferred from skeletal δ18O is dominated by a 9.5-year period, and may constitute a characteristic signal of the Leeuwin Current. On long-terms coral
skeletal δ18O indicates a near-continuous increase of SST at Ningaloo Reef over one century. The skeletal δ18O time series was checked for the presence of seasonal cooling events resulting from major volcanic eruptions. An ∼1 °C cooling
is evident following the eruption of Pinatubo in 1991, which reproduces the results of previous investigations. However, only
weak or no signals can be related to the eruptions of Krakatau (1883) and Agung (1963).
Received: 9 November 1998 / Accepted: 12 April 1999 相似文献
26.
Late Quaternary rapid climate change in northern Chile 总被引:2,自引:0,他引:2
Analyses of terrigenous sediments from the Chilean continental slope off the southern border of the Atacama desert (27.5°S), focusing on illite crystallinity and the Fe:Al ratio of the sediments, reveal a high-frequency variability of the position of the Southern Westerlies, which is very similar to the coeval short-term climatic events known from Greenland ice cores and from North Atlantic sediments. Besides showing dominantly precession-driven variability in precipitation over the Andes, these analyses also reveal rapid changes in weathering intensity along the Chilean Coastal Range during the last 80,000 years. These rapid changes occur at much shorter timescales than the 19–100 kyr orbital forcing of the Milankovitch cycles. 相似文献
27.
Sediment dynamics and geohazards off Uruguay and the de la Plata River region (northern Argentina and Uruguay) 总被引:1,自引:0,他引:1
Sebastian Krastel Gerold Wefer Till J. J. Hanebuth Andrew A. Antobreh Tim Freudenthal Benedict Preu Tilmann Schwenk Michael Strasser Roberto Violante Daniel Winkelmann 《Geo-Marine Letters》2011,31(4):271-283
The continental margin off Uruguay and northern Argentina is characterized by high fluvial input by the de la Plata River and a complex oceanographic regime. Here we present first results from RV Meteor Cruise M78/3 of May?CJuly 2009, which overall aimed at investigating sediment transport processes from the coast to the deep sea by means of hydroacoustic and seismic mapping, as well as coring using conventional tools and the new MARUM seafloor drill rig (MeBo). Various mechanisms of sediment instabilities were identified based on geophysical and core data, documenting particularly the continental slope offshore Uruguay to be locus of submarine landsliding. Individual landslides are relatively small with volumes <2km3. Gravitational downslope sediment transport also occurs through the prominent Mar del Plata Canyon and several smaller canyons. The canyons originate at a midslope position, and the absence of buried upslope continuations strongly suggests upslope erosion as main process for canyon evolution. Many other morphological features (e.g., slope-parallel scarps with scour geometries) and abundant contourites in a 35-m-long MeBo core reveal that sediment transport and erosion are controlled predominantly by strong contour currents. Despite numerous landslide events, their geohazard potential is considered to be relatively small, because of their small volumes and their occurrence at relatively deep water depths of more than 1,500?m. 相似文献
28.
The reconstruction of the climatic history during the past several hundred years requires a sufficient geographical coverage
of combined climate proxy series. Especially in order to identify causal connections between the atmosphere and the ocean,
inclusion of marine records into composite climate time series is of fundamental importance. We present two skeletal δ
18O chronologies of coral skeletons of Diploria labyrinthiformis from Bermuda fore-reef sites covering periods in the nineteenth and twentieth centuries and compare them with instrumental
temperature data. Both time series are demonstrated to display sea-surface temperature (SST) variability on inter-annual to
decadal time scales. On the basis of a specific modern δ
18O vs instrumental SST calibration we reconstruct a time series of SST anomalies between AD 1350 and 1630 covering periods
during the Little Ice Age. The application of the coral δ
18O vs temperature relationship leads to estimates of past SST variability which are comparable to the magnitude of modern variations.
Parallel to δ
18O chronologies we present time series of skeletal bulk density. Coral δ
18O and skeletal density reveal a strong similarity during Little Ice Age, confirming the reliability of both proxy climate
indicators. The past coral records, presented in this study, share features with a previously published climate proxy record
from Bermuda and a composite time series of reconstructed Northern Hemisphere summer temperatures. The coral proxy data presented
here represent a valuable contribution to elucidate northern Atlantic subtropical climate variation during the past several
centuries.
Received: 9 November 1998 / Accepted: 13 September 1999 相似文献
29.
Y. A. Moustafa J. Pätzold Y. Loya G. Wefer 《International Journal of Earth Sciences》2000,88(4):742-751
We present a study based on X-ray chronologies and the stable isotopic composition of fossil Porites spp. corals from the northern Gulf of Aqaba (Red Sea) covering the mid-Holocene period from 5750 to 4450 14C years BP (before present). The stable oxygen and carbon isotopic compositions of five specimens reveal regular annual periodicities.
Compared with modern Porites spp. from the same environment, the average seasonal δ
18O amplitude of the fossil corals is higher (by ca. 0.35–0.60‰), whereas annual growth rates are lower (by ca. 3.5 to 2 mm/year).
This suggests stronger seasonality of sea surface temperatures and increased variability of the oxygen isotopic composition
of the sea water due to changes in the precipitation and evaporation regime during the mid-Holocene. Most likely, summer monsoon
rains reached the northern end of the Red Sea at that time. Average annual coral growth rates are diminished probably due
to an increased input and resuspension of terrestrial debris to the shallow marine environment during more humid conditions.
Our results corroborate published reports of paleodata and model simulations suggesting a northward migration of the African
monsoon giving rise to increased seasonalities during the mid-Holocene over northeastern Africa and Arabia.
Received: 4 January 1999 / Accepted: 13 September 1999 相似文献
30.
High-resolution benthic oxygen isotope and XRF (Fe and Ca) records from Site 1085 drilled in the Mid-Cape basin (ODP Leg 175) are used to investigate global climate changes during the Late Miocene in relation to Messinian geological events. The cyclic fluctuations of the time series at Site 1085 enable us to establish a reliable chronology for the time interval 7.3–4.7 Ma. Spectral analysis of the δ18O record indicates that the 41-kyr period of orbital obliquity dominates the Late Miocene record. A global climate record was extracted from the oxygen isotopic composition of benthic foraminifera. Both long- and short-term variabilities in the climate record are discussed in terms of sea-level and deep-water temperature changes. The time interval 7.3–6.25 Ma characterized by low-amplitude δ18O variations is followed by a period marked by maximum in the δ18O values (6.25–5.57 Ma). At about 5.56 Ma, a rapid decrease in δ18O values is documented that may reflect a warming of deep-water temperature associated with a global warming period. Comparison between the timing of the oceanic isotope events and the chronology of the Mediterranean Salinity Crisis suggest that global eustatic processes were not essential in the Mediterranean Salinity Crisis history. From our data, we infer that the global warmth documented in the Early/mid-Pliocene probably started during the Late Miocene (at 5.55 Ma). At the same time, the onset of evaporite deposition in the central basin of the Mediterranean Sea took place. Sharp changes in the sedimentation rates, mainly driven by terrigenous input at this site, are observed during the Messinian Stage. 相似文献