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  • 1
    Language: English
    In: Geology, August, 2014, Vol.42(8), p.663(4)
    Description: In the Nordic seas, conversion of inflowing warm Atlantic surface water to deep cold water through convection is closely linked with climate. During the last glacial period, climate underwent rapid millennial-scale variability known as Dansgaard-Oeschger (DO) events, consisting of warm interstadials and cold stadials. Here we present the first benthic foraminiferal Mg/Ca-[delta][sup.18]O record from the Nordic seas in order to reconstruct the ocean circulation on DO time scales. The record confirms that convection similar to modern took place in the Nordic seas during interstadials with cold bottom water temperatures (BWTs) close to modern temperatures. The results show gradual and pronounced BWT increases of 2-5[degrees]C during stadials, indicating a stop or near stop in convection. The BWT peaks are followed by an abrupt drop in temperature at the onset of interstadials, indicating the abrupt start of convection and renewed generation of cold deep water. The rise in BWT during stadials confirms earlier interpretations of subsurface inflow of warm Atlantic water below a halocline reaching 〉1.2 km water depth. The results suggest that warm Atlantic water never ceased to flow into the Nordic seas during the glacial period; inflow at the surface during the Holocene and warm interstadials changed to subsurface and intermediate inflow during cold stadials. Our results suggest that it is the vertical shifts in the position of the warm Atlantic water that cause the abrupt surface warnings. doi:10.1130/G35579.1
    Keywords: Water Circulation -- Research ; Water Circulation -- Reports ; Surface Water -- Research
    ISSN: 0091-7613
    E-ISSN: 19432682
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  • 2
    Language: English
    In: Palaeogeography, Palaeoclimatology, Palaeoecology, 2011, Vol.309(1), pp.17-32
    Description: In an attempt to document the palaeoecological affinities of individual extant and extinct dinoflagellate cysts, Late Pliocene and Early Pleistocene dinoflagellate cyst assemblages have been compared with geochemical data from the same samples. Mg/Ca ratios of were measured to estimate the spring–summer sea-surface temperatures from four North Atlantic IODP/DSDP sites. Currently, our Pliocene–Pleistocene database contains 204 dinoflagellate cyst samples calibrated to geochemical data. This palaeo-database is compared with modern North Atlantic and global datasets. The focus lies in the quantitative relationship between Mg/Ca-based (i.e. spring–summer) sea-surface temperatures (SST ) and dinoflagellate cyst distributions. In general, extant species are shown to have comparable spring–summer SST ranges in the past (SST ) and today (SST from World Ocean Atlas 2005, Locarnini et al., 2006), demonstrating that our new approach is valid for inferring spring–summer SST ranges for extinct species. For example, represents SST values between 10 and 15 °C when it exceeds 30% of the assemblage, and exceeds 15% when SST values are between 18.6 and 23.5 °C. However, comparing Pliocene and Pleistocene SST values with present day summer values for the extant suggests a greater tolerance of higher temperatures in the past. This species occupies more than 5% of the assemblage at SST values of 11.6–17.9 °C in the Pliocene and Pleistocene, whereas present day summer SSTs are around − 1.7 to 6.9 °C. This observation questions the value of as reliable indicator of cold waters in older deposits, and may explain its bipolar distribution. ► Dinocyst palaeoecology was investigated from four North Atlantic locations. ► Pliocene and Pleistocene dinocyst assemblages were calibrated to SST of . . ► Our data confirm and augment the spring–summer SST distribution of extant species. ► We provide spring–summer SST ranges of 16 extinct species. ► is tolerant of high SSTs and questioned as cold-water indicator.
    Keywords: Dinocyst ; Transfer Function ; Palaeoecology ; North Atlantic ; Mg/Ca Geochemistry ; Quaternary ; Geology
    ISSN: 0031-0182
    E-ISSN: 1872-616X
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  • 3
    Language: English
    In: Earth and Planetary Science Letters, 2010, Vol.292(1), pp.89-97
    Description: Quantifying the spatial and temporal sea surface temperature (SST) and salinity changes of the Indo-Pacific Warm Pool is essential to understand the role of this region in connection with abrupt climate changes particularly during the last deglaciation. In this study we reconstruct SST and seawater O of the tropical eastern Indian Ocean for the past 40,000 years from two sediment cores (GeoB 10029-4, 1°30′S, 100°08′E, and GeoB 10038-4, 5°56′S, 103°15′E) retrieved offshore Sumatra. Our results show that annual mean SSTs increased about 2–3 °C at 19,000 years ago and exhibited southern hemisphere-like timing and pattern during the last deglaciation. Our SST records together with other Mg/Ca-based SST reconstructions around Indonesia do not track the monsoon variation since the last glacial period, as recorded by terrestrial monsoon archives. However, the spatial SST heterogeneity might be a result of changing monsoon intensity that shifts either the annual mean SSTs or the seasonality of towards the warmer or the cooler season at different locations. Seawater O reconstructions north of the equator suggest fresher surface conditions during the last glacial and track the northern high-latitude climate change during the last deglaciation. In contrast, seawater O records south of the equator do not show a significant difference between the last glacial period and the Holocene, and lack Bølling-Allerød and Younger Dryas periods suggestive of additional controls on annual mean surface hydrology in this part of the Indo-Pacific Warm Pool.
    Keywords: Indian Ocean ; Indonesia ; Foraminifera ; Oxygen Isotopes ; Mg/Ca ; Seawater Δ18o ; Geology ; Physics
    ISSN: 0012-821X
    E-ISSN: 1385-013X
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  • 4
    Language: English
    In: Palaeogeography, Palaeoclimatology, Palaeoecology, 2010, Vol.289(1), pp.33-43
    Description: The monsoon system represents one of the basic elements of global atmospheric circulation. Its evolution and variability over long periods of geologic time play a significant role in our understanding of global climate. In this study, we focus on the Late Miocene interval from 10 Ma to 6 Ma, a period of postulated profound ecological and environmental shifts in East and South Asia. The combined approach of measuring planktonic foraminiferal Mg/Ca ratios and stable oxygen isotopes from Ocean Drilling Program (ODP) Site 1146 enabled us to reconstruct temperature independent seawater O (i.e. proxy for sea surface salinity) variations in order to reconstruct the hydrography in the northern South China Sea. Located offshore the Pearl (Zhujiang) River, or its predecessor, the location of ODP Site 1146 is considered to provide a most sensitive record for detecting potential changes in freshwater input/river run-off as a result of changes in continental humidity, and hence changes in East Asian summer monsoon (EASM) climate. Local seawater O reconstructions reveal that monsoon development during the Late Miocene can be regarded as an abrupt weakening in the EASM around 7.5 Ma. We suggest that an EASM weakening was most likely the driving force for decreasing aridity in East and South Asia at 8–6 Ma, leading to widespread ecosystem changes in East and South Asia.
    Keywords: East Asian Monsoon ; South China Sea ; Late Miocene ; Foraminifera ; Stable Isotopes ; Mg/Ca Ratios ; Geology
    ISSN: 0031-0182
    E-ISSN: 1872-616X
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  • 5
    In: Geophysical Research Letters, 16 November 2014, Vol.41(21), pp.7684-7693
    Description: Modern variability in upwelling off southern Indonesia is strongly controlled by the Australian‐Indonesian monsoon and the El Niño–Southern Oscillation, but multidecadal to centennial‐scale variations are less clear. We present high‐resolution records of upper water column temperature, thermal gradient, and relative abundances of mixed layer‐ and thermocline‐dwelling planktonic foraminiferal species off southern Indonesia for the past two millennia that we use as proxies for upwelling variability. We find that upwelling was generally strong during the Little Ice Age (LIA) and weak during the Medieval Warm Period (MWP) and the Roman Warm Period (RWP). Upwelling is significantly anticorrelated to East Asian summer monsoonal rainfall and the zonal equatorial Pacific temperature gradient. We suggest that changes in the background state of the tropical Pacific may have substantially contributed to the centennial‐scale upwelling trends observed in our records. Our results implicate the prevalence of an El Niño‐like mean state during the LIA and a La Niña‐like mean state during the MWP and the RWP. Upwelling variations off southern Indonesia over the past 2000 yearsUpwelling was strong during Little Ice Age and weak during Medieval Warm PeriodENSO may have contributed to changes in upwelling off southern Indonesia
    Keywords: Upwelling ; Indonesia ; Late Holocene
    ISSN: 0094-8276
    E-ISSN: 1944-8007
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  • 6
    Language: English
    In: Palaeogeography, Palaeoclimatology, Palaeoecology, 15 March 2017, Vol.470, pp.81-108
    Description: Dinoflagellate cysts (dinocysts) are widely used as tracers of sea surface conditions in late Quaternary marine records. However, paleoenvironmental reconstructions across the Pliocene–Pleistocene climatic transition and beyond are limited because the hydrologic conditions influencing assemblage compositions may not have a modern analogue, and the ecological optima of extinct dinocyst species are not well known. From a study of two cored sites in the central and eastern North Atlantic, we bypass these issues by statistically analyzing the variations in dinocyst assemblage composition and comparing the results directly to paleoecological parameters (δ O , δ O , and geochemical proxies for sea surface temperature [SST]) derived from the planktonic foraminifer recovered from the same samples as the dinocysts. Through canonical correspondence analysis we demonstrate the co-variation of seasonality and dinocyst paleoproductivity. We show that is a cold tolerant species with an optimum SST between 12 and 14 °C. We extend the use of as an indicator of transitional climatic conditions to the Pliocene, we offer evidence for the correlation of and to high seasonality, and we reiterate an apparent link between and eastern North Atlantic water masses. Finally, we confirm that is cold-tolerant rather than a strictly cold-water indicator, that ? is a cold-intolerant species favoring outer neritic environments, and that and are both warm-water species.
    Keywords: Geology
    ISSN: 0031-0182
    E-ISSN: 1872-616X
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  • 7
    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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  • 8
    Language: English
    In: PLOS ONE, 2013
    Description: The early Late Pliocene (3.6 to ~3.0 million years ago) is the last extended interval in Earth's history when atmospheric CO2 concentrations were comparable to today's and global climate was warmer. Yet a severe global glaciation during marine isotope stage (MIS) M2 interrupted this phase of global warmth ~3.30 million years ago, and is seen as a premature attempt of the climate system to establish an ice-age world. Here we propose a conceptual model for the glaciation and deglaciation of MIS M2 based on geochemical and palynological records from five marine sediment cores along a Caribbean to eastern North Atlantic transect. Our records show that increased Pacific-to-Atlantic flow via the Central American Seaway weakened the North Atlantic Current and attendant northward heat transport prior to MIS M2. The consequent cooling of the northern high latitude oceans permitted expansion of the continental ice sheets during MIS M2, despite near-modern atmospheric CO2 concentrations. Sea level drop during this glaciation halted the inflow of Pacific water to the Atlantic via the Central American Seaway, allowing the build-up of a Caribbean Warm Pool. Once this warm pool was large enough, the Gulf Stream–North Atlantic Current system was reinvigorated, leading to significant northward heat transport that terminated the glaciation. Before and after MIS M2, heat transport via the North Atlantic Current was crucial in maintaining warm climates comparable to those predicted for the end of this century.
    Keywords: Earth And Environmental Sciences ; Ma ; Reconstruction ; Pleistocene ; Mg/Ca Ratios ; Atlantic-Ocean ; Oxygen Isotopes ; Overturning Circulation ; Thermohaline Circulation ; Dinoflagellate Cysts ; Sea-Level
    ISSN: 1932-6203
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  • 9
    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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  • 10
    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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