Dierk Hebbeln

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.

FLUCTUATIONS OF THE SVALBARD–BARENTS SEA ICE SHEET DURING THE LAST 150 000 YEARS

Abstract On Spitsbergen, western Svalbard, three major glacial advances have been identified during the Weichselian. All three reached the continental shelf west of the Svalbard archipelago. Radiocarbon, luminescence and amino acid dating of interbedded interstadial and interglacial sediments indicate that these glacial advances have Early (Isotope Stage 5d), Middle (Stage 4), and Late Weichselian ages (Stage 2). An additional, more local, advance has been dated to Isotope Stage 5b. The Late Weichselian ice sheet expanded across the entire Barents Sea. However, in the south-western Barents Sea, the Late Weichselian till is the only till above Eemian sediments, indicating that the Early- and Middle Weichselian ice advances were restricted to the Svalbard archipelago and the northern Barents Sea. A major problem with the onshore sites is the dating of events beyond the range of the radiocarbon method. To overcome this, the onshore record has been correlated with marine cores from the continental slope and the deep-sea west of Svalbard, where a chronology has been established by oxygen isotope stratigraphy. Ice rafted detritus (IRD) was used as the main monitor of glaciation. The IRD record closely mirrors the glaciation history as interpreted from the onshore sections. During the Late Weichselian, the largest IRD peak occurred during deglaciation, a pattern also postulated for the earlier events. Given this, the results from the marine cores indicate that the ages for the first glacial advances during the Weichselian were a few thousand years older than interpreted from the onshore stratigraphy.

Basin‐wide particulate carbon flux in the Atlantic Ocean: Regional export patterns and potential for atmospheric CO2 sequestration

Particle flux data from 27 sites in the Atlantic Ocean have been compiled in order to determine regional variations in the strength and efficiency of the biological pump and to quantify carbon fluxes over the ocean basin, thus estimating the potential oceanic sequestration of atmospheric CO2. An algorithm is derived relating annual particulate organic carbon (POC) flux to primary production and depth that yields variations in the export ratio (ER = POC flux/primary production) at 125 m of between 0.08 and 0.38 over the range of production from 50 to 400 g C m−2 yr−1. Significant regional differences in changes of the export ratio with depth are related to the temporal stability of flux. Sites with more pulsed export have higher export ratios at 125 m but show more rapid decreases of POC flux with depth, resulting in little geographic variation in fluxes below ∼3000 m. The opposing effects of organic carbon production and calcification on ΔpCO2 of surface seawater are considered to calculate an “effective carbon flux” at the depth of the euphotic zone and at the base of the winter mixed layer. POC flux at the base of the euphotic zone integrated over the Atlantic Ocean between 65°N and 65°S amounts to 3.14 Gt C yr−1. Of this, 5.7% is remineralized above the winter mixed layer and thus does not contribute to CO2 sequestration on climatically relevant timescales. The effective carbon flux, termed Jeff, amounts to 2.47 Gt C yr−1 and is a measure of the potential sequestration of atmospheric CO2 for the area considered. A shift in the composition of sedimenting particles (seen in a decrease of the opal:carbonate ratio) is seen across the entire North Atlantic, indicating a basin-wide phenomenon that may be related to large-scale changes in climatic forcing.

Distribution of planktic foraminifera at the ice margin in the Arctic (Fram Strait)

Planktic foraminifera were collected in net tows (>63 μm) along two east-west transects at 78 ° and 80 °N in the Fram Strait (northern Nordic Seas) at five concurrent depth intervals in the upper 200 m of the water column. In the ice-free zone, absolute abundances in the upper 200 m of the water column up to 500 individuals/m3 were obtained. Along the ice margin the absolute abundances reached up to 1250 ind/m3, while in the ice-covered areas not more than 50 ind/m3 have been found. Surprisingly, these strong variations in abundance are not accompanied by significant changes in faunal assemblages between the ice-free and the ice-covered regions. Neogloboquadrina pachyderma is the dominant species in most samples, making up to 60% of the total faunal assemblage. Globigerina quinqueloba contributes 38%, and all other species combined make up only 2% of the total fauna. Both, absolute and relative abundances of the collected foraminifera are strongly dependent on the mesh size used. Using smaller mesh sizes provides a wider species spectrum, which is of special interest for paleoceanographic reconstructions in the Nordic Seas, where fossil assemblages >150 μm often consist of only one species. The specific hydrographic conditions in the Fram Strait, with warm Atlantic waters underlying cold Polar surface waters, result in a rather atypical depth distribution of the planktic foraminifera. All species seem to prefer the temperate Atlantic waters (between 50 m and 200 m). Thus, the species composition as well as geochemical tracers do not reflect the surface ocean conditions, which again has potentially interesting implications for paleoceanographic reconstructions.

Surface sediment distribution along the Chilean continental slope related to upwelling and productivity

Abstract The coastal upwelling system of the Peru–Chile Current belongs to the most productive regions in the world oceans. In spite of this remarkable fact only very little is known about the sediment distribution in its southern part off the coast of Chile. To increase the knowledge about this region a multiparameter study of the surface sediment distribution along the Chilean continental slope between 27 and 42°S has been carried out. Detailed analyses of sedimentary data (TOC and carbonate contents, δ13Corg, δ15N), of the species composition of planktic foraminifera and of the stable isotope composition of the two planktic foraminifera species Neogloboquadrina pachyderma (dex.) and Globigerina bulloides reveal a close relation to environmental conditions in the region. The pattern of a southward increase of surface water productivity known from satellite data is closely reflected in the organic carbon and δ15N data. Information about the water mass distribution, especially about varying influences of upwelling waters and warmer Subtropical Surface Waters, can be derived from the species composition of the planktic foraminifera, while the stable isotope data provide additional clues about water mass characteristics. High productivity along the Chilean margin north of 40°S is sustained by coastal upwelling, while further to the south advection of nutrients with the Antarctic Circumpolar Current and a significant riverine input fuel the highest productivity found in the study area. This data set will provide the base for future paleoclimatic and paleoceanographic reconstructions of the Southeast Pacific region.

PALEOCEANOGRAPHY OF THE LAST INTERGLACIAL/GLACIAL CYCLE IN THE POLAR NORTH ATLANTIC

Abstract Using to its position close to the sites of waxing and waning of the major northern hemisphere ice sheets during the Late Quaternary climatic cycles, the Polar North Atlantic plays a key role in driving global change. Therefore, analyzing the present-day sedimentation processes and reconstructing the Late Quaternary paleoceanography in this region has been a focus of major scientific interest through the last fifteen years. This paper aims to combine the vast amount of new results about modern sediment fluxes, surface sediment distribution and the paleoceanographic record and to present a comprehensive overview of the paleoceanography in the Polar North Atlantic through the last 200,000 years, which is based on the basinwide analysis of carbonate and ice-rafted detritus records. During the last fifteen years the CLIMAP paleoceanographic view of the glacial Polar North Atlantic as an almost permanently isolated sea covered by heavy sea ice throughout the year has been shifted to a much more dynamic view of the environmental conditions. A meridional current system similar to the present-day conditions secured the exchange of water, ice and heat between the Polar North Atlantic and the Alantic and Arctic Oceans. Although affected by variations in strength and intensity this meridional current pattern resulted in an almost permanent presence of at least some seasonally ice-free areas, with all the consequences for e.g. marine life and deep water formation. In addition, the development of the continental ice sheets, namely the Fennoscandian and the Barents Sea ice sheets, is closely related to this dynamic circulation pattern. During the so-called Nordway events the pronounced inflow of temperate waters from the south provided moisture for the growth of the ice sheets. The most prominent of these events (Stage 6, Stage 4 and Substage 3.1/2) ended in major glaciations, reflected in the terrestrial sequences and in the deep-sea IRD records.

Late Quaternary precessional cycles of terrigenous sediment input off the Norte Chico, Chile (27.5°S) and palaeoclimatic implications

Abstract The palaeoclimatic conditions during the Last Glacial Maximum (LGM) of southern South America and especially latitudinal shifts of the southern westerly wind belt are still discussed controversially. Longer palaeoclimatic records covering the Late Quaternary are rare. A particularly sensitive area to Late Quaternary climatic changes is the Norte Chico, northern Chile, because of its extreme climatic gradients. Small shifts of the present climatic zonation could cause significant variations of the terrestrial sedimentary environment which would be recorded in marine terrigenous sediments. To unveil the history of shifting climatic zones in northern Chile, we present a sedimentological study of a marine sediment core (GeoB 3375-1) from the continental slope off the Norte Chico (27.5°S). Sedimentological investigations include bulk- and silt grain-size determinations by sieving, Atterberg separation, and detailed SediGraph analyses. Additionally, clay mineralogical parameters were obtained by X-ray diffraction methods. The 14C-dated core, covering the time span from approximately 10,000 to 120,000 cal. yr B.P., consists of hemipelagic sediments. Terrigenous sedimentological parameters reveal a strong cyclicity, which is interpreted in terms of variations of sediment provenance, modifications of the terrestrial weathering regimes, and modes of sediment input to the ocean. These interpretations imply cyclic variations between comparatively arid climates and more humid conditions with seasonal precipitation for northern Chile (27.5°S) through the Late Quaternary. The cyclicity of the terrigenous sediment parameters is strongly dominated by precessional cycles. For the palaeoclimatic signal, this means that more humid conditions coincide with maxima of the precession index, as e.g. during the LGM. Higher seasonal precipitation for this part of Chile is most likely derived from frontal winter rain of the Southern Westerlies. Thus, the data presented here favour not only an equatorward shift of this atmospheric circulation system during the LGM, but also precession-controlled latitudinal movements throughout the Late Quaternary. Precessional forcing of latitudinal movements of the westerly atmospheric circulation system may be conceivable through teleconnections to the Northern Hemisphere monsoonal system in the Atlantic Ocean region.

The impact of sediment provenance on barium-based productivity estimates

Abstract Biogenic barium in marine sediments has been suggested to be a reliable proxy of export productivity from the surface ocean and algorithms have been developed to link these properties. However, problems arise when the proposed algorithms are applied to predominantly terrigenous sediments. A major source of error is incorrect estimates of the terrigenous Ba/Al ratio in normative calculations of the amount of biogenic barium in the sediment. Compared to an often used “global average” Ba/Al ratio, much better results can be obtained by estimating the terrigenous Ba/Al ratio from exponential regression of the Ba/Al ratios of surface sediments obtained from continental slope transects. This method has been applied to surface sediments from the Chilean continental slope. The calculated regional terrigenous Ba/Al ratios could be verified with purely terrigenous samples from Chilean rivers. The resulting accumulation rates of biogenic barium on the Chilean continental slope reliably reproduce the regional pattern of primary productivity in the southern Peru–Chile Current, indicating the potential of biogenic barium as a useful (paleo)productivity proxy.

Late Quaternary paleoceanography in the Fram Strait

Four sediment cores from the Fram Strait (78°N) have been studied to reconstruct the paleoceanography of this major connection of the Arctic Ocean to the world ocean. Back to oxygen isotope stage 5 (128 kyr ago) the stratigraphic interpretation of the cores is based on oxygen isotope and accelerator mass spectrometry data; the identification of oxygen isotope stage 6 is based on paleomagnetic and coccolith data. Parameters indicative for contrasting environmental conditions such as high input of ice-rafted detritus (IRD) or seasonally open waters display characteristic sequences by which the paleoce anography of this region can be reconstructed for the last 180 kyr. The geographic setting of Fram Strait and the sediment data indicate a permanent meridional circulation pattern throughout the last 180 kyr, as it is typical, for example, for the present-day circulation. Depending on the strength of advection, Atlantic waters appeared in Fram Strait as surface waters, resulting in seasonally ice-free conditions, or as subsurface water masses, underlying polar waters perenially covered by ice. Sustained periods of seasonally ice-free waters were largely restricted to interglacial stages 5.5, 5.1, and the Holocene. However, a few short events characterized by seasonally ice-free conditions at 78°N are recorded during glacial stages as well, for example, stage 6, late stage 3, and stage 2 (“Nordway” events). These events provide for supply of moisture, and therefore they are closely related to the history of the European Arctic ice sheets. Strong input of IRD, reflecting severe glaciations on surrounding landmasses, occurred during most of stage 6, in early stage 3, and during stage 2. Variations in IRD input allow differentiation between three major source areas: (1) Svalbard/Barents Sea during major glacier advances in stages 2 to 5; (2) most likely Siberia during stage 6; and (3) Fennoscandia during the short events in glacial periods marked by seasonal open waters. Thus the circulation pattern in the Fram Strait remained rather stable throughout the last 180 kyr, while variations in the sediment composition are mainly due to the intensity of Atlantic water advection and to the prevailing IRD input pathway.

Flux of ice-rafted detritus from sea ice in the Fram Strait

Abstract A three-year particle flux record from the eastern Fram Strait, between Greenland and Svalbard, revealed a rather untypical seasonal flux pattern compared to other particle flux studies from the Nordic Seas. In the eastern Fram Strait this pattern is characterised by a sudden four- to six-fold increase of the particle flux in January, when no daylight is available to support any biological productivity. Comparison with sea-ice distribution maps led to the conclusion that the sudden increase in the flux is due to ice-rafted detritus released from sea ice, which originated from the Svalbard archipelago and from the northern Barents Sea. Detailed grain size analyses of the silt fraction indicated the >10 μm fraction of the lithogenic matter to be clearly enriched due to IRD input. Even more important is the observation that lithogenic material >40 μm occurs exclusively during the ice-rafting event and, therefore, appears to be a suitable indicator for IRD transported on sea ice. Thus, in addition to coarse IRD (e.g. >500 μm ), which is mainly derived from icebergs, the analysis of fine IRD (>40 μm ) in deep-sea sediments can be used to reconstruct paleo-sea-ice extensions.