Gerold Wefer

Isotope paleontology: growth and composition of extant calcareous species

Abstract Isotope paleontology uses the isotopic composition of fossil remains of organisms to make inferences about the physical surroundings of growth of the organisms (especially temperature), and to obtain clues about life history and modes of growth. In calcareous fossils, oxygen isotopes are mainly used in the former, and carbon isotopes in the latter. However, since physical surroundings and organism response are intimately associated, both types of information are contained in each of the isotopic signals. To explore the potential of isotope paleontology, and to provide a basis for reconstruction, a broad range of extant organisms has been studied, taking the pioneering work of Epstein and associates as a starting point. Results are summarized for a representative sampling of these studies, with emphasis on work at the laboratories of the authors, from the mid-seventies to the present. The organisms considered are nannoplankton, benthic algae, planktonic and benthic deep-sea foraminifera, “larger” foraminifera, sponges, corals, bryozoans, polychaetes, arthropods, bivalves, gastropods, cephalopods, and vertebrates (fish otoliths). The survey broadly suggests that, regarding oxygen isotopes, materials tend to be precipitated close to equilibrium with the surrounding seawater (as postulated by Urey), and that for carbon isotopes disequilibrium is the rule. Life spans, growth rates, differential seasonal growth, and age of reproductive activity can be extracted under favorable circumstances from individual shells and skeletal parts. In detail, the interpretation of isotope records of individual shells is quite complicated, and simple models will fail to give satisfactory results in many or most cases.

Warming of the tropical Atlantic Ocean and slowdown of thermohaline circulationduring the last deglaciation

Evidence for abrupt climate changes on millennial and shorter timescales is widespread in marine and terrestrial climate records. Rapid reorganization of ocean circulation is considered to exert some control over these changes, as are shifts in the concentrations of atmospheric greenhouse gases. The response of the climate system to these two influences is fundamentally different: slowing of thermohaline overturn in the North Atlantic Ocean is expected to decrease northward heat transport by the ocean and to induce warming of the tropical Atlantic, whereas atmospheric greenhouse forcing should cause roughly synchronous global temperature changes. So these two mechanisms of climate change should be distinguishable by the timing of surface-water temperature variations relative to changes in deep-water circulation. Here we present a high-temporal-resolution record of sea surface temperatures from the western tropical North Atlantic Ocean which spans the past 29,000 years, derived from measurements of temperature-sensitive alkenone unsaturation in sedimentary organic matter. We find significant warming is documented for Heinrich event H1 (16,900–15,400 calendar years bp) and the Younger Dryas event (12,900–11,600 cal. yr bp), which were periods of intense cooling in the northern North Atlantic. Temperature changes in the tropical and high-latitude North Atlantic are out of phase, suggesting that the thermohaline circulation was the important trigger for these rapid climate changes.

New chronology for the late Paleocene thermal maximum and its environmental implications

The late Paleocene thermal maximum (LPTM) is associated with a brief, but intense, interval of global warming and a massive perturbation of the global carbon cycle. We have developed a new orbital chronology for Ocean Drilling Program (ODP) Site 690 (Weddell Sea, Southern Ocean) by using spectral analysis of high-resolution geochemical records. The LPTM interval spans 11 precessional cycles yielding a duration of 210 to 220 k.y. The δ 13 C anomaly associated with the LPTM has a magnitude of about −2.5‰ to −3‰; we show that about −2‰ of the excursion occurs within two steps that each were less than 1000 yr in duration. The remainder developed through a series of steps over ∼52 k.y. The timing of these steps is consistent with a series of nearly catastrophic releases of methane from gas hydrates, punctuated by intervals of relative equilibria between hydrate dissociation and carbon burial. Further, we are able to correlate the records between ODP Sites 690 and 1051 (western North Atlantic) on the scale of 21 k.y. cycles, which demonstrates that the details of the δ 13 C excursion are recognizable between distant sites. Comparison of cycle records at Sites 690 and 1051 suggests that sediment representing the interval ∼30 k.y. just prior to and at the onset of the LPTM are missing in the latter location. This unconformity probably resulted from slope failure accompanying methane hydrate dissociation within 10 k.y. of the start of the LPTM.

Holocene rainfall variability in southern Chile: a marine record of latitudinal shifts of the Southern Westerlies.

Geochemical and clay mineral parameters of a high accumulation marine sediment core from the Chilean continental slope (41‡S) provide a 7700 yr record of rainfall variability in southern Chile related to the position of the Southern Westerlies. We especially use the iron content, measured with a time-resolution of ca. 10 yr on average, of 14 Caccelerator mass spectrometry dated marine sediments as a proxy for the relative input of iron-poor Coastal Range and iron-rich Andean source rocks. Variations in this input are most likely induced by rainfall changes in the continental hinterland of the core position. Based on these interpretations, we find a pronounced rainfall variability on multicentennial to millennial time-scales, superimposed on generally more arid conditions during the middle Holocene (7700 to 4000 cal yr B.P.) compared to the late Holocene (4000 to present). This variability and thus changes in the position of the Southern Westerlies are first compared to regional terrestrial paleoclimate data-sets from central and southern Chile. In order to derive possible wider implications and forcing mechanisms of the Holocene latitudinal shifts of the Southern Westerlies, we then compare our data to ice-core records from both tropical South America and coastal Antarctica. These records show similar bands of variability centered at ca. 900 and 1500 yr. Comparisons of band pass filters suggest a close connection of shifts of the Southern Westerlies to changes within the tropical climate system. The correlation to climate conditions in coastal Antarctica shows a more complicated picture with a phase shift at the beginning of the late Holocene coinciding with the onset of the modern state of El Nin ‹ o-Southern Oscillation system. The presented data provide further evidence that the well known millennial-scale climate variability during the last glacial continued throughout the Holocene. fl 2001 Elsevier Science B.V. All rights reserved.

Early diagenesis of organic matter from sediments of the eastern subtropical Atlantic: evidence from stable nitrogen and carbon isotopes

Stable isotopes of sedimentary nitrogen and organic carbon are widely used as proxy variables for biogeochemical parameters and processes in the water column. In order to investigate alterations of the primary isotopic signal by sedimentary diagenetic processes, we determined concentrations and isotopic compositions of inorganic nitrogen (IN), organic nitrogen (ON), total nitrogen (TN), and total organic carbon (TOC) on one short core recovered from sediments of the eastern subtropical Atlantic, between the Canary Islands and the Moroccan coast. Changes with depth in concentration and isotopic composition of the different fractions were related to early diagenetic conditions indicated by pore water concentrations of oxygen, nitrate, and ammonium. Additionally, the nature of the organic matter was investigated by Rock-Eval pyrolysis and microscopic analysis. A decrease in ON during aerobic organic matter degradation is accompanied by an increase of the 15 N/ 14 N ratio. Changes in the isotopic composition of ON can be described by Rayleigh fractionation kinetics which are probably related to microbial metabolism. The influence of IN depleted in 15 N on the bulk sedimentary (TN) isotope signal increases due to organic matter degradation, compensating partly the isotopic changes in ON. In anoxic sediments, fixation of ammonium between clay lattices results in a decrease of stable nitrogen isotope ratio of IN and TN. Changes in the carbon isotopic composition of TOC have to be explained by Rayleigh fractionation in combination with different remineralization kinetics of organic compounds with different isotopic composition. We have found no evidence for preferential preser- vation of terrestrial organic carbon. Instead, both TOC and refractory organic carbon are dominated by marine organic matter. Refractory organic carbon is depleted in 13 C compared to TOC. Copyright

Seasonal particle flux in the Bransfield Strait, Antarctica

Time-series sediment traps were deployed at 494 and 1588 m water depth in Bransfield Strait from 1 December, 1983 to 25 November, 1984. During austral summer (December and January) total flux was more than 1.5 g m−2 day−1 to both water depths, while during all other months 1984 flux was between 10 and 1000 times lower. The annual total flux to the deeper trap (1588 m) was 110 g m−2. The flux of the two most productive months was 97% of the total. Biogenic components (carbonate, POM and opaline silica) accounted for about 67% in the upper trap and 50% in the lower one. The remaining portion of the material collected were lithogenic particles. The transfer of the particles is mainly through the fecal pellets of krill.

Seasonal patterns of vertical particle flux in equatorial and coastal upwelling areas of the eastern Atlantic

Eight time-series sediment traps were deployed on moored arrays off Mauritania (Cap Blanc), in the northern and southern Guinea Basin, and off Namibia (Walvis Ridge). The highest total fluxes (66.8 and 59.1 g m−2y−1), as well as carbonate (28.7 and 32.9 g m−2y−1) and biogenic opal fluxes (5.4 and 8.2 g m−2y−1) were recorded at the two coastal sites, off Mauritania and off Namibia, respectively. Intermediate rates were recorded north and low rates south of the equator. Surprisingly, carbonate-producing organisms dominated total fluxes at all sites, whereas biogenic opal was only a minor contributor. The most distinct flux maxima were in July–August both at the Cap Blanc and the northern Guinea Basin site (2°N). Carbonate fluxes also peaked during that period. In contrast, highest opal fluxes were measured there in late winter/early spring. At the Cap Blanc site, high lithogenic fluxes correlate well with fluxes of biogenic components. At both equatorial sites, prominent flux maxima were observed in boreal spring, attributed to the southermost penetration of the ITCZ. These fluxes were characterized by a relatively high lithogenic contribution. South of the equator (2°S), sedimentation decreased rapidly to almost zero after May when the ITCZ started to migrate northwards. Seasonality was most expressed at the Walvis Ridge site, where a distinct bi-modal flux pattern occurred (June–July and October–November). In austral spring, elevated fluxes were determined for a relatively long period when wind-driven upwelling is strongest and the Namibia upwelling cell shows the greatest extension to the west approaching the trap position. Contribution of lithogenic material to total fluxes was very low at this site.

The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems

In order to understand current and future patterns of climate variability and change, we need to know how climate varied in the past and which physical forcing mechanisms led to these climatic changes. Direct observations of climate indicators such as temperature and rainfall reach back about 150 years. To extend this observational record, we rely on information from environmental paleoclimatic proxy records that have been extracted from natural archives of past climate variability.

Sahel megadroughts triggered by glacial slowdowns of Atlantic meridional overturning

[1] The influence of the large-scale ocean circulation on Sahel rainfall is elusive because of the shortness of the observational record. We reconstructed the history of eolian and fluvial sedimentation on the continental slope off Senegal during the past 57,000 years. Our data show that abrupt onsets of arid conditions in the West African Sahel were linked to cold North Atlantic sea surface temperatures during times of reduced meridional overturning circulation associated with Heinrich Stadials. Climate modeling suggests that this drying is induced by a southward shift of the West African monsoon trough in conjunction with an intensification and southward expansion of the midtropospheric African Easterly Jet.

Planktonic foraminifera as recorders of past surface-water stratification

The δ 18 O difference between shallow- and deep-living planktonic foraminifera is a proxy for the stratification of surface waters. In Holocene sediments from the Atlantic this difference increases from about 0‰ in subpolar regions to ∼3‰ in the tropics. The δ 18 O values of the shallow dwellers Globigerinoides sacculifer and Neogloboquadrina pachyderma mainly reflect surface-water conditions. The calcification depth of the deep-living species Globorotalia truncatulinoides (right-coiling) is affected by the stratification of the water column. In the subtropics, shell formation of G. truncatulinoides (right-coiling) begins in weakly stratified surface waters. In the tropics, the initial shell is secreted below the mixed layer. This hydrographically induced difference in calcification depth allows the use of the isotopic difference between G. truncatulinoides and the shallow species as a proxy for the stratification of surface waters. The Δδ 18 O between G. truncatulinoides and G. sacculifer from tropical gravity Core GeoB 1523-1 was significantly lower during isotope stage 2. This suggests that the glacial temperature stratification of western equatorial Atlantic surface waters was significantly reduced relative to the Holocene.