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  • 1
    Language: English
    In: Proceedings of the National Academy of Sciences of the United States of America, 22 May 2018, Vol.115(21), pp.5365-5370
    Description: The Pacific hosts the largest oxygen minimum zones (OMZs) in the world ocean, which are thought to intensify and expand under future climate change, with significant consequences for marine ecosystems, biogeochemical cycles, and fisheries. At present, no deep ventilation occurs in the North Pacific due to a persistent halocline, but relatively better-oxygenated subsurface North Pacific Intermediate Water (NPIW) mitigates OMZ development in lower latitudes. Over the past decades, instrumental data show decreasing oxygenation in NPIW; however, long-term variations in middepth ventilation are potentially large, obscuring anthropogenic influences against millennial-scale natural background shifts. Here, we use paleoceanographic proxy evidence from the Okhotsk Sea, the foremost North Pacific ventilation region, to show that its modern oxygenated pattern is a relatively recent feature, with little to no ventilation before six thousand years ago, constituting an apparent Early-Middle Holocene (EMH) threshold or "tipping point." Complementary paleomodeling results likewise indicate a warmer, saltier EMH NPIW, different from its modern conditions. During the EMH, the Okhotsk Sea switched from a modern oxygenation source to a sink, through a combination of sea ice loss, higher water temperatures, and remineralization rates, inhibiting ventilation. We estimate a strongly decreased EMH NPIW oxygenation of ∼30 to 50%, and increased middepth Pacific nutrient concentrations and carbon storage. Our results () imply that under past or future warmer-than-present conditions, oceanic biogeochemical feedback mechanisms may change or even switch direction, and () provide constraints on the high-latitude North Pacific's influence on mesopelagic ventilation dynamics, with consequences for large oceanic regions.
    Keywords: Holocene ; North Pacific ; Intermediate Water ; Oxygen Minimum Zone ; Stable Isotopes
    ISSN: 00278424
    E-ISSN: 1091-6490
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  • 2
    Language: English
    In: Palaeogeography, Palaeoclimatology, Palaeoecology, 01 April 2017, Vol.471, pp.144-156
    Description: The internal development of the tropical West Pacific Warm Pool and its interaction with high latitude ocean regions on geological timescales is only poorly constrained. Based on two newly recovered sediment cores from the southeastern margin of the West Pacific Warm Pool (northern and southern Manihiki Plateau), we provide new aspects on the dynamically interacting ocean circulation at surface, subsurface, thermocline, and deep thermocline levels during the Pleistocene (~ 2.5–0.5 Ma). Notably, the variability of thermocline and deep thermocline (~ 150–400 m water depth) foraminiferal Mg/Ca-based temperatures with up to ~ 6 °C amplitude variations exceeds those at shallower depths (down to ~ 120 m) with only ~ 2–3 °C temperature variations. A major gradual reorganization of the West Pacific Warm Pool oceanography occurred during the transitional time period of ~ 1.7–1.35 Ma. Prior to ~ 1.7 Ma, pronounced meridional and latitudinal gradients in sea-surface to subsurface ocean properties point to the eastward displacement of the West Pacific Warm Pool boundaries, with the South Pacific Convergence Zone being shifted further northeastward across Manihiki Plateau. Simultaneously, the low amplitude variations of thermocline and deep thermocline temperatures refer to an overall deep and stable thermocline. The meridional and zonal gradients in sea-surface and subsurface ocean properties within the West Pacific Warm Pool reveal a pronounced change after 1.5 Ma, leading to a more southward position of the warm South Pacific Convergence Zone between ~ 1.35–0.9 Ma and ~ 0.75–0.5 Ma. Synchronous to the changes in the upper ocean, the deeper water masses experienced high amplitude variations in temperature, most prominently since ~ 1.5 Ma. This and the dynamically changing thermocline were most likely associated to the impact of southern-sourced mode waters, which might have developed coincidently with the emergence of the East Pacific Cold Tongue and high latitude sea-surface cooling.
    Keywords: Geology
    ISSN: 0031-0182
    E-ISSN: 1872-616X
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  • 3
    Language: English
    In: Science, 08 September 2000, Vol.289(5485), pp.1698-1699
    Description: Accurate reconstructions of past climatic conditions on Earth are important for modeling future climate change. Researchers use climate "proxies" from deep-sea sediments, corals, or ice cores to reconstruct these past climatic conditions, but as Nürnberg explains in this Perspective, different techniques often give different answers. A relatively new technique, Mg/Ca paleothermometry, is a particularly promising tool, especially in conjunction with oxygen isotope measurements on the same planktonic foraminiferal specimens. The power of this method is exemplified by the study by Lea et al.
    Keywords: Environmental studies -- Environmental sciences -- Climate change ; Physical sciences -- Earth sciences -- Hydrology ; Environmental studies -- Atmospheric sciences -- Climatology ; Physical sciences -- Earth sciences -- Geography ; Physical sciences -- Earth sciences -- Geography ; Physical sciences -- Earth sciences -- Hydrology ; Physical sciences -- Chemistry -- Chemical properties ; Physical sciences -- Earth sciences -- Geology ; Physical sciences -- Earth sciences -- Geology ; Physical sciences -- Earth sciences -- Hydrology
    ISSN: 00368075
    E-ISSN: 10959203
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  • 4
    Language: English
    In: Palaeogeography, Palaeoclimatology, Palaeoecology, 2011, Vol.310(3), pp.191-205
    Description: High-resolution records of ice-rafted debris (IRD) and oxygen isotope records spanning an E–W-trending transect across the Okhotsk Sea unravel the marine and terrestrial cryogenic history of NE-Siberia over the last 350 kyr. IRD, predominantly dispersed basin-wide by sea-ice, shows lowest fluxes during interglacial periods implying a reduced and seasonal sea-ice coverage. Highest IRD accumulation rates are observed during glacial and deglacial periods with a more extended, but highly dynamic ice cover. Although being rather synchronous, IRD fluxes are on average higher in the western Okhotsk Sea than in the eastern part, pointing to a persistent but mobile, particle-supplying sea-ice cover even during full glacial conditions, presumably less dense in the eastern parts. MIS 6 is exceptional in this respect: Asynchronous fluxes of IRD, which vary spatially, reflect rapid paleoclimatic and paleo-glaciomarine changes. IRD accumulation rates were by factor 2–3 higher during MIS 6 as compared to the last glacial maximum, and the IRD depositional center shifted from the western Okhotsk Sea (early MIS 6) toward Kamchatka (late MIS 6), synchronous to a distinct change in the IRD mineral composition. Both, the characteristic composition of late MIS 6 IRD originating from the Sredinny Mountain Range of Kamchatka and their significantly enhanced accumulation rates refer to intensified iceberg dispersal across the eastern part of the Okhotsk Sea at ~ 138 ka, ~ 135 ka, ~ 129 ka, and ~ 128 ka BP. This scenario affords the presence of extended mountain glaciers protruding down to sea level on the western side of Kamchatka. Anomalously light planktonic stable oxygen isotopes during MIS 6.3, Termination II and MIS 5.5–5.4 suggest significant freshwater supply related to the westward drainage of Kamchatka glaciers. The intensified Kamchatka glaciation observed during late MIS 6 was repeated during MIS 3. Iceberg discharges into the eastern Okhotsk Sea are observed at ~ 60 ka, ~ 51 ka, ~ 42 ka, ~ 38 ka, ~ 36 ka, and ~ 31 ka, and may partly correspond to N-Atlantic Heinrich Events. ► We unravel the late Pleistocene cryogenic history of NE-Siberia. ► We present records of ice-rafted debris and oxygen isotopes from the Okhotsk Sea. ► We develop conceptions of sea-ice and iceberg dynamics through time. ► We propose an intensified Kamchatka mountain glaciation during late MIS 6 and MIS 3.
    Keywords: Okhotsk Sea ; Ice Rafted Debris ; Kamchatka Glaciation ; Foraminiferal Oxygen Isotopes ; Marine Isotope Stage 6 ; Geology
    ISSN: 0031-0182
    E-ISSN: 1872-616X
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  • 5
    In: Geophysical Research Letters, 28 July 2013, Vol.40(14), pp.3538-3543
    Description: Very few age controls exist for Quaternary deposits over the vast territory of the East Russian Arctic, which hampers dating of major environmental changes in this area and prevents their correlation to climatic changes in the Arctic and Pacific marine domains. We report a newly identified ~177 ka old Rauchua tephra, which has been dispersed over an area of 〉1,500,000 km and directly links terrestrial paleoenvironmental archives from Arctic Siberia with marine cores in the northwest Pacific, thus permitting their synchronization and dating. The Rauchua tephra can help to identify deposits formed in terrestrial and marine environments during the oxygen isotope stage 6.5 warming event. Chemical composition of volcanic glass from the Rauchua tephra points to its island‐arc origin, while its spatial distribution singles out the Kamchatka volcanic arc as a source. The Rauchua tephra represents a previously unknown, large (magnitude 〉6.5) explosive eruption from the Kamchatka volcanic arc. The Rauchua tephra links Arctic and Pacific paleoclimatic archives The Rauchua tephra marks MIS6.5 warming event in distant sequences Tephra composition and dispersal area suggest a huge eruption from Kamchatka
    Keywords: Tephra ; Arctic ; Kamchatka ; Explosive Eruption ; Pacific
    ISSN: 0094-8276
    E-ISSN: 1944-8007
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  • 6
    Language: English
    In: Palaeogeography, Palaeoclimatology, Palaeoecology, 2010, Vol.298(3), pp.311-318
    Description: The long term history of terrigenous flux to the Gulf of Mexico via the Mississippi River is hardly known. We here present geochemical and sedimentological data to approximate the varying Mississippi River sediment influx into the northeastern Gulf of Mexico (GoM) over the last six glacial-interglacial cycles (MIS 1 to 14). Our study is based on the IMAGES sediment core MD02-2576 that was recovered from the DeSoto Canyon and is located ~ 200 km south to the recent Mississippi River delta and ~ 150 km east of the recent coastline of Florida. Concentrations of siliciclastic elements in bulk sediment samples were estimated from XRF scanning and calibrated by single bulk XRF-analyses. Elemental ratios of the sedimentary record correspond to ratios from the Mississippi River catchment rather than to the core close Alabama and Mobile River catchments. The siliciclastic major element potassium (K) with enhanced surface concentrations in the northwestern Mississippi River catchment shows varying occurrence downcore and here serves as a proxy for Mississippi River sediment discharge variability. Changes in sedimentation rate and magnetic susceptibility further support the variations in Mississippi River influx. Our data were compared with Mississippi River terrestrial archives in the form of loess and terrace deposits that back up our interpretations of enhanced glacial phase Mississippi River influx triggered by strengthened fluvial river runoff and changing fluvial and ice sheet dynamics. Mississippi River influx was at a maximum during glacial MIS 2/3, MIS 8 and MIS 10. Late glacial MIS 6 deviates from this pattern being a period of reduced Mississippi River influx at the core location, probably due to a westward shift of the Mississippi River delta.
    Keywords: Mississippi Sediment Discharge ; Mississippi Flood Dynamics ; Gulf of Mexico ; Glacial-Interglacial Cycles ; Xrf-Scanning ; Geology
    ISSN: 0031-0182
    E-ISSN: 1872-616X
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  • 7
    Language: English
    In: Earth and Planetary Science Letters, 15 May 2012, Vol.331-332, pp.360-363
    Keywords: Geology ; Physics
    ISSN: 0012-821X
    E-ISSN: 1385-013X
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  • 8
    Language: English
    In: Marine Micropaleontology, December 2015, Vol.121, pp.52-69
    Description: Benthic foraminiferal assemblage compositions, foraminiferal and sedimentological proxies were analyzed in core MD01-2415 from the northern slope of the Okhotsk Sea to compare variations of productivity and oxygen minimum zone (OMZ) intensity during the latest marine isotope stage (MIS) 12–11c and latest MIS 2–1. The benthic assemblages reveal close similarity between the two climatic cycles. The absence of benthic assemblages during the latest MIS 12 and the presence of the low-productivity assemblage during the latest MIS 2 suggest the disappearance of the OMZ. This regime was related to almost perennial ice cover with periods of active ice rafting during the latest MIS 12, while it was attributed to prolonged seasonal ice cover, low surface productivity and enhanced formation of well-oxygenated water masses in the Okhotsk Sea during the latest MIS 2. In deglacial times, the OMZ gradually intensified, as evidenced by the high-productivity assemblage during the early termination (T) TV and early TI and the low oxygen-tolerant assemblage during the late TV and late TI. The orbitally forced similar rises of the global sea level during TV and TI caused a large offshore supply of organic matter. Synchronously, nutrients from the melting sea ice and shelf erosion promoted high surface (mainly carbonate) productivity. As a result, a high and sustained flux of particulate, degraded and refractory organic matter enhanced oxygen consumption in bottom waters and sediments, leading to the similar gradual OMZ intensifications. The assemblage also points to expansion of oxygen-depleted water mass from the North Pacific into the Okhotsk Sea, fostering the OMZ intensifications. The phytodetritus-related assemblage indicates weakening of the OMZ during full interglacial times of MIS 11c and MIS 1. After stabilization of the global sea level, nutrients were mainly delivered by regional upwelling and fluvial discharge, favoring increased biogenic opal and carbonate production in the surface water, similar to the present. In this way, moderate to high (although less than deglacial) and pulsed flux of predominantly particulate organic matter caused the weakening of the oxygen consumption and OMZ. Notably, during MIS 11c, the benthic assemblage with the dominance of the taxa with dissolution-resistant tests, such as , and , reflects carbonate dissolution events in sediments. These events might have been resulted from an interruption of the local surface carbonate production and inflow of more carbonate-corrosive water masses from the North Pacific driven by a drawdown of the global ocean carbonate saturation.
    Keywords: Benthic Foraminifera ; Organic Matter Flux ; Oxygen Minimum Zone ; Downslope Supply ; Carbonate Dissolution Events ; Okhotsk Sea ; Geology
    ISSN: 0377-8398
    E-ISSN: 1872-6186
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  • 9
    Language: English
    In: Global and planetary change, 2013, Vol.111, pp.77-87
    Description: Subtropical Gyres are an important constituent of the ocean–atmosphere system due to their capacity to store vast amounts of warm and saline waters. Here we decipher the sensitivity of the (sub)surface North Atlantic Subtropical Gyre with respect to orbital and millennial scale climate variability between ~140 and 70ka, Marine Isotope Stage (MIS) 5. Using (isotope) geochemical proxy data from surface and thermocline dwelling foraminifers from Blake Ridge off the west coast of North America (ODP Site 1058) we show that the oceanographic development at subsurface (thermocline) level is substantially different from the surface ocean. Most notably, surface temperatures and salinities peak during the penultimate deglaciation (Termination II) and early MIS 5e, implying that subtropical surface ocean heat and salt accumulation might have resulted from a sluggish northward heat transport. In contrast, maximum thermocline temperatures are reached during late MIS 5e when surface temperatures are already declining. We argue that the subsurface warming originated from intensified Ekman downwelling in the Subtropical Gyre due to enhanced wind stress. During MIS 5a-d a tight interplay of the subtropical upper ocean hydrography to high latitude millennial-scale cold events can be observed. At Blake Ridge, the most pronounced of these high latitude cold events are related to surface warming and salt accumulation in the (sub)surface. Similar to Termination II, heat accumulated in the Subtropical Gyre probably due to a reduced Atlantic Meridional Overturning Circulation. Additionally, a southward shift and intensification of the subtropical wind belts lead to a decrease of on-site precipitation and enhanced evaporation, coupled to intensified gyre circulation. Subsequently, the northward advection of this warm and saline water likely contributed to the fast resumption of the overturning circulation at the end of these high latitude cold events. ; p. 77-87.
    Keywords: Wind ; Evaporation ; Saline Water ; Cold ; Isotopes ; Coasts ; Surface Temperature ; Latitude ; Heat ; Climate
    ISSN: 0921-8181
    Source: AGRIS (Food and Agriculture Organization of the United Nations)
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  • 10
    Language: English
    In: International Journal of Earth Sciences, 2018, Vol.107(2), pp.505-515
    Description: The ultimate, possibly geodynamic control and potential impact of changes in circulation activity and salt discharge of Mediterranean outflow waters (MOW) on Atlantic meridional overturning circulation have formed long-standing objectives in paleoceanography. Late Pliocene changes in the distal advection of MOW were reconstructed on orbital timescales for northeast Atlantic DSDP/ODP sites 548 and 982 off Brittany and on Rockall Plateau, supplemented by a proximal record from Site U1389 west off Gibraltar, and compared to Western Mediterranean surface and deep-water records of Alboran Sea Site 978. From ~3.43 to 3.3 Ma, MOW temperatures and salinities form a prominent rise by 2–4 °C and ~3 psu, induced by a preceding and coeval rise in sea surface and deep-water salinity and increased summer aridity in the Mediterranean Sea. We speculate that these changes triggered an increased MOW flow and were ultimately induced by a persistent 2.5 °C cooling of Indonesian Through-Flow waters. The temperature drop resulted from the northward drift of Australia that crossed a threshold value near 3.6–3.3 Ma and led to a large-scale cooling of the eastern subtropical Indian Ocean and in turn, to a reduction of African monsoon rains. Vice versa, we show that the distinct rise in Mediterranean salt export after ~3.4 Ma induced a unique long-term rise in the formation of Upper North Atlantic Deep Water, that followed with a phase lag of ~100 ky. In summary, we present evidence for an interhemispheric teleconnection of processes in the Indonesian Gateways, the Mediterranean and Labrador Seas, jointly affecting Pliocene climate.
    Keywords: Global climate change ; Paleoceanography ; Late Pliocene ; Atlantic salinity balance ; Shift of ocean gateway ; Mediterranean climate
    ISSN: 1437-3254
    E-ISSN: 1437-3262
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