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  • 1
    In: Paleoceanography, December 2011, Vol.26(4), pp.n/a-n/a
    Description: Multidecadal variations in Atlantic sea surface temperatures (SST) influence the climate of the Northern Hemisphere. However, prior to the instrumental time period, information on multidecadal climate variability becomes limited, and there is a particular scarcity of sufficiently resolved SST reconstructions. Here we present an eastern tropical North Atlantic reconstruction of SSTs based on foraminiferal Mg/Ca ratios that resolves multidecadal variability over the past 1700 years. Spectral power in the multidecadal band (50 to 70 years period) is significant over several time intervals suggesting that the Atlantic Multidecadal Oscillation (AMO) has been influencing local SST. Since our data exhibit high scatter the absence of multidecadal variability in the remaining record does not exclude the possibility that SST variations on this time scale might have been present without being detected in our data. Cooling by ∼0.5°C takes place between about AD 1250 and AD 1500; while this corresponds to the inception of the Little Ice Age (LIA), the end of the LIA is not reflected in our record and SST remains relatively low. This transition to cooler SSTs parallels the previously reconstructed shift in the North Atlantic Oscillation toward a low pre‐20th century mean state and possibly reflects common solar forcing. Monsoon season SST is reconstructed for the past 3 millennia Over the past 1700 years, several intervals show multidecadal SST variability Late medieval cooling amounts to approximately 0.5 degree Celsius
    Keywords: Mg/Ca ; Eastern Tropical Atlantic ; Multidecadal Variability ; Sea Surface Temperature
    ISSN: 0883-8305
    E-ISSN: 1944-9186
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  • 2
    In: Nature, 2010, Vol.466(7303), p.226
    Description: The Sahara Desert is the largest source of mineral dust in the world (1). Emissions of African dust increased sharply in the early 1970s (ref. 2), a change that has been attributed mainly to drought in the Sahara/Sahel region (2) caused by changes in the global distribution of sea surface temperature (3,4). The human contribution to land degradation and dust mobilization in this region remains poorly understood (5-11), owing to the paucity of data that would allow the identification of long-term trends in desertification (12). Direct measurements of airborne African dust concentrations only became available in the mid-1960s from a station on Barbados (2) and subsequently from satellite imagery since the late 1970s: they do not cover the onset of commercial agriculture in the Sahel region ~170 years ago (11,13,14). Here we construct a 3,200-year record of dust deposition off northwest Africa by investigating the chemistry and grain-size distribution of terrigenous sediments deposited at a marine site located directly under the West African dust plume. With the help of our dust record and a proxy record for West African precipitation (15) we find that, on the century scale, dust deposition is related to precipitation in tropical West Africa until the seventeenth century. At the beginning of the nineteenth century, a sharp increase in dust deposition parallels the advent of commercial agriculture in the Sahel region. Our findings suggest that human-induced dust emissions from the Sahel region have contributed to the atmospheric dust load for about 200 years.
    Keywords: Agriculture -- Environmental Aspects ; Terrigenous Sediments -- Research;
    ISSN: 0028-0836
    E-ISSN: 14764687
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  • 3
    Language: English
    In: Earth and Planetary Science Letters, July 15, 2014, Vol.398, p.1(10)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.epsl.2014.04.034 Byline: James A. Collins, Enno Schefu[sz], Aline Govin, Stefan Mulitza, Ralf Tiedemann Abstract: The past climate evolution of southwestern Africa is poorly understood and interpretations of past hydrological changes are sometimes contradictory. Here we present a record of leaf-wax [delta]D and [delta]C13 taken from a marine sediment core at 23[degrees]S off the coast of Namibia to reconstruct the hydrology and C.sub.3 versus C.sub.4 vegetation of southwestern Africa over the last 140a000 years (140 ka). We find lower leaf-wax [delta]D and higher [delta]C13 (more C.sub.4 grasses), which we interpret to indicate wetter Southern Hemisphere (SH) summer conditions and increased seasonality, during SH insolation maxima relative to minima and during the last glacial period relative to the Holocene and the last interglacial period. Nonetheless, the dominance of C.sub.4 grasses throughout the record indicates that the wet season remained brief and that this region has remained semi-arid. Our data suggest that past precipitation increases were derived from the tropics rather than from the winter westerlies. Comparison with a record from the Congo Basin indicates that hydroclimate in southwestern Africa has evolved in antiphase with that of central Africa over the last 140 ka. Author Affiliation: (a) MARUM - Center for Marine Environmental Sciences, University of Bremen, D-28359 Bremen, Germany (b) Alfred Wegener Institute for Polar and Marine Research, Am Alten Hafen 26, D-27568 Bremerhaven, Germany Article History: Received 30 September 2013; Revised 16 April 2014; Accepted 22 April 2014 Article Note: (miscellaneous) Editor: G.M. Henderson
    Keywords: History -- Analysis ; Climate -- Analysis ; Interglacial Periods -- Analysis ; Rain -- Analysis ; Hydrology -- Analysis
    ISSN: 0012-821X
    Source: Cengage Learning, Inc.
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  • 4
    Language: English
    In: Earth and Planetary Science Letters, June 1, 2012, Vol.333-334, p.317(15)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.epsl.2012.04.005 Byline: Jorg Lippold (a), Stefan Mulitza (b), Gesine Mollenhauer (b), Stefan Weyer (c), David Heslop (d), Marcus Christl (e) Keywords: .sup.231Pa/.sup.230Th; AMOC; boundary scavenging; North Atlantic Deep Water; LGM; African margin Abstract: The.sup.231Pa/.sup.230Th method is a promising tool to reconstruct Ocean circulation over the past Glacial-Interglacial cycle. However, marine particle flux may constrain the applicability of this ratio as a direct quantitative proxy for the strength of Atlantic Meridional Overturning Circulation (AMOC) by influencing the fractionation between the in situ produced.sup.231Pa and.sup.230Th in ocean water. Here we present.sup.231Pa/.sup.230Th down-core profiles from high particle flux areas off Namibia and Senegal covering the past [approximately equal to]35ka. The.sup.231Pa/.sup.230Th profiles at these sites show very different responses to temporal variations of particle fluxes and to changes in water masses. Our results show that sedimentary.sup.231Pa/.sup.230Th in the Eastern Atlantic margin is linked to particle flux, but controlled primarily by the mode of the AMOC. Our data suggest that during the past [approximately equal to]30ka the high productivity Eastern margin was not capable of importing and storing significant amounts of.sup.231Pa from the open Ocean. Consequently, the applicability of the.sup.231Pa/.sup.230Th proxy to reconstruct past ocean circulation is not hampered by this potentially additional.sup.231Pa sink. Author Affiliation: (a) University of Heidelberg, Institute of Environmental Physics, 69120 Heidelberg, Germany (b) MARUM, University of Bremen, Institute of Geoscience, 28359 Bremen, Germany (c) University of Frankfurt, Institute of Geoscience, 60438 Frankfurt, Germany (d) Australian National University, Research School of Earth Sciences, Canberra ACT 0200, Australia (e) ETH Zurich, Laboratory of Ion Beam Physics, 8093 Zurich, Switzerland Article History: Received 17 June 2011; Revised 29 March 2012; Accepted 6 April 2012 Article Note: (miscellaneous) Editor: G. Henderson
    Keywords: Ocean Circulation ; Thermohaline Circulation
    ISSN: 0012-821X
    Source: Cengage Learning, Inc.
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  • 5
    Language: English
    In: Earth and Planetary Science Letters, 2011, Vol.308(1), pp.161-171
    Description: Understanding natural climate variability in the North Atlantic region is essential not only to assess the sensitivity of atmosphere–ocean climate signal exchange and propagation, but also to help distinguish between natural and anthropogenic climate change. The North Atlantic Oscillation is one of the controlling modes in recent variability of atmosphere–ocean linkages and ice/freshwater fluxes between the Polar and North Atlantic Ocean. Through these processes the NAO influences water mass formation and the strength of the Atlantic Meridional Overturning circulation and thereby variability in ocean heat transport. However, the impact of the NAO as well as other forcing mechanisms on multidecadal timescales such as total solar irradiance on Eastern North Atlantic Central Water production, central water circulation, and climate signal propagation from high to low latitudes in the eastern subpolar and subtropical basins remains uncertain. Here we use a 1200 yr long benthic foraminiferal Mg/Ca based temperature and oxygen isotope record from a ~ 900 m deep sediment core off northwest Africa to show that atmosphere–ocean interactions in the eastern subpolar gyre are transferred at central water depth into the eastern boundary of the subtropical gyre. Further we link the variability of the NAO (over the past 165 yrs) and solar irradiance (Late Holocene) and their control on subpolar mode water formation to the multidecadal variability observed at mid-depth in the eastern subtropical gyre. Our results show that eastern North Atlantic central waters cooled by up to ~ 0.8 ± 0.7 °C and densities decreased by σ = 0.3 ± 0.2 during positive NAO years and during minima in solar irradiance during the Late Holocene. The presented records demonstrate the sensitivity of central water formation to enhanced atmospheric forcing and ice/freshwater fluxes into the eastern subpolar gyre and the importance of central water circulation for cross-gyre climate signal propagation during the Late Holocene. ► 1200 yr. long paired Mg/Ca–δ O central water record in the eastern North Atlantic. ► Multidecadal variability of Eastern North Atlantic Central Water. ► Sensitivity of ENACW formation to subpolar gyre sea surface temperatures. ► Cross-gyre climate signal transfer of NAO and solar irradiance into ENACW.
    Keywords: Benthic Mg/Ca Paleothermometry ; North Atlantic Central Water Formation ; Late Holocene ; Solar Minima — Lia ; Nao ; Climate Signal Transfer ; Geology ; Physics
    ISSN: 0012-821X
    E-ISSN: 1385-013X
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  • 6
    Language: English
    In: Palaeogeography, Palaeoclimatology, Palaeoecology, Dec 1, 2014, Vol.415, p.28(9)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.palaeo.2013.12.005 Byline: Cristiano M. Chiessi, Stefan Mulitza, Jeroen Groeneveld, Juliana B. Silva, Marilia C. Campos, Marcio H.C. Gurgel Abstract: Our understanding of the centennial-scale variability of the Brazil Current (BC) during the late Holocene is elusive because of the lack of appropriate records. Here we used the Mg/Ca and oxygen isotopic composition of planktonic foraminifera from two marine sediment cores collected at 27[degrees]S and 33[degrees]S off southeastern South America to assess the late Holocene variability in the upper water column of the BC. Our results show in phase fluctuations of up to 3[degrees]C in sea surface temperatures (SST), and 0.8a[degrees] in oxygen isotopic composition of surface sea water, a proxy for relative sea surface salinity (SSS). Time-series analyses of our records indicate a cyclicity with a period of ca. 730yr. We suggest that the observed cyclicity reflects variability in the strength of the BC associated to changes in the Atlantic meridional overturning circulation (AMOC). Positive (negative) SST and SSS anomalies are related to a strong (weak) BC and a weak (strong) AMOC. Moreover, periods of peak strength in the BC occur synchronously to a weak North Brazil Current, negative SST anomalies in the high latitudes of the North Atlantic, and positive (negative) precipitation anomalies over southeastern South America (equatorial Africa), further corroborating our hypothesis. This study shows a tight coupling between the variability of the BC and the high latitudes of the North Atlantic mediated by the AMOC even under late Holocene boundary conditions. Article History: Received 1 August 2013; Revised 29 November 2013; Accepted 4 December 2013
    Keywords: Holocene Paleogeography -- Analysis ; Thermohaline Circulation -- Analysis ; Precipitation (Meteorology) -- Analysis
    ISSN: 0031-0182
    Source: Cengage Learning, Inc.
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  • 7
    In: Geophysical Research Letters, February 2011, Vol.38(4), pp.n/a-n/a
    Description: Two SST records based on Mg/Ca of (pink) from the continental slope off West Africa at 15°N and 12°N shed new light on the thermal bipolar seesaw pattern in the northeastern tropical Atlantic during periods of reduced Atlantic Meridional Overturning Circulation (AMOC) associated with Heinrich stadials H1 to H6. The two records indicate that the latitudinal position of the bipolar seesaw's zero‐anomaly line, between cooling in the North and warming in the South, gradually shifted southward from H6 to H1. A conceptual model is presented that aims to provide a physically consistent mechanism for the southward migration of the seesaw's fulcrum. The conceptual model suggests latitudinal movements of the Intertropical Convergence Zone, driven by a combination of orbital‐forced changes in the meridional temperature gradient within the realm of the Hadley cell and the expansion of the Northern Hemisphere cryosphere, as a major factor.
    Keywords: Bipolar Seesaw ; Tropical Atlantic ; Mg/Ca ; Sea Surface Temperature ; Heinrich Stadials ; West Africa
    ISSN: 0094-8276
    E-ISSN: 1944-8007
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  • 8
    Language: English
    In: Palaeogeography, Palaeoclimatology, Palaeoecology, 01 December 2014, Vol.415, pp.28-36
    Description: Our understanding of the centennial-scale variability of the Brazil Current (BC) during the late Holocene is elusive because of the lack of appropriate records. Here we used the Mg/Ca and oxygen isotopic composition of planktonic foraminifera from two marine sediment cores collected at 27°S and 33°S off southeastern South America to assess the late Holocene variability in the upper water column of the BC. Our results show in phase fluctuations of up to 3 °C in sea surface temperatures (SST), and 0.8‰ in oxygen isotopic composition of surface sea water, a proxy for relative sea surface salinity (SSS). Time-series analyses of our records indicate a cyclicity with a period of ca. 730 yr. We suggest that the observed cyclicity reflects variability in the strength of the BC associated to changes in the Atlantic meridional overturning circulation (AMOC). Positive (negative) SST and SSS anomalies are related to a strong (weak) BC and a weak (strong) AMOC. Moreover, periods of peak strength in the BC occur synchronously to a weak North Brazil Current, negative SST anomalies in the high latitudes of the North Atlantic, and positive (negative) precipitation anomalies over southeastern South America (equatorial Africa), further corroborating our hypothesis. This study shows a tight coupling between the variability of the BC and the high latitudes of the North Atlantic mediated by the AMOC even under late Holocene boundary conditions.
    Keywords: South Atlantic ; Late Quaternary ; Western Boundary Current ; Stable Oxygen Isotopes ; Mg/Ca ; Planktonic Foraminifera ; Geology
    ISSN: 0031-0182
    E-ISSN: 1872-616X
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  • 9
    In: Geophysical Research Letters, September 2010, Vol.37(17), pp.n/a-n/a
    Description: Central waters of the North Atlantic are fundamental for ventilation of the upper ocean and are also linked to the strength of the Atlantic Meridional Overturning Circulation (AMOC). Here, we show based on benthic foraminiferal Mg/Ca ratios, that during times of enhanced melting from the Laurentide Ice Sheet (LIS) between 9.0–8.5 thousand years before present (ka) the production of central waters weakened the upper AMOC resulting in a cooling over the Northern Hemisphere. Centered at 8.54 ± 0.2 ka and 8.24 ± 0.1 ka our dataset records two ∼150‐year cooling events in response to the drainage of Lake Agassiz/Ojibway, indicating early slow‐down of the upper AMOC in response to the initial freshwater flux into the subpolar gyre (SPG) followed by a more severe weakening of both the upper and lower branches of the AMOC at 8.2 ka. These results highlight the sensitivity of regional North Atlantic climate change to the strength of central‐water overturning and exemplify the impact of both gradual and abrupt freshwater fluxes on eastern SPG surface water convection. In light of the possible future increase in Greenland Ice Sheet melting due to global warming these findings may help us to better constrain and possibly predict future North Atlantic climate change.
    Keywords: Paleoceanography ; Early Holocene ; Mg/Ca Paleothermometry
    ISSN: 0094-8276
    E-ISSN: 1944-8007
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  • 10
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 November 2016, Vol.192, pp.149-165
    Description: Paleoenvironmental studies based on terrigenous biomarker proxies from sediment cores collected close to the mouth of large river systems rely on a proper understanding of the processes controlling origin, transport and deposition of biomarkers. Here, we contribute to the understanding of these processes by analyzing long-chain alkanes from the Amazon River system. We use the δD composition of long-chain alkanes from river bed sediments from the Amazon River and its major tributaries, as well as marine core-top samples collected off northeastern South America as tracers for different source areas. The δ C composition of the same compounds is used to differentiate between long-chain alkanes from modern forest vegetation and petrogenic organic matter. Our δ C results show depleted δ C values (−33 to −36‰) in most samples, indicating a modern forest source for most of the samples. Enriched values (−31 to −33‰) are only found in a few samples poor in organic carbon indicating minor contributions from a fossil petrogenic source. Long-chain alkane δD analyses show more depleted values for the western tributaries, the Madeira and Solimões Rivers (−152 to −168‰), while alkanes from the lowland tributaries, the Negro, Xingu and Tocantins Rivers (−142 to −154‰), yield more enriched values. The alkane δD values thus reflect the mean annual isotopic composition of precipitation, which is most deuterium-depleted in the western Amazon Basin and more enriched in the eastern sector of the basin. Samples from the Amazon estuary show a mixed long-chain alkane δD signal from both eastern lowland and western tributaries. Marine core-top samples underlying the Amazon freshwater plume yield δD values similar to those from the Amazon estuary, while core-top samples from outside the plume showed more enriched values. Although the variability in the river bed data precludes quantitative assessment of relative contributions, our results indicate that long-chain alkanes from the Amazon estuary and plume represent an integrated signal of different regions of the onshore basin. Our results also imply that alkanes are not extensively remineralized during transport and that the signal at the Amazon estuary and plume includes refractory compounds derived from the western sector of the Basin. These findings will aid in the interpretation of plant wax-based records of marine sediment cores collected from the adjacent ocean.
    Keywords: Amazon River ; Amazon Plume ; Biomarker Proxies ; Compound-Specific Δd ; Compound-Specific Δ13c ; N-Alkanes ; Paleoclimate Proxies ; Particulate Organic Carbon Transport ; Particulate Organic Carbon Deposition ; Plant Waxes ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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