149 results found
Struve T, Wilson DJ, van de Flierdt T, et al., 2020, Middle Holocene expansion of Pacific Deep Water into the Southern Ocean, Proceedings of the National Academy of Sciences, Vol: 117, Pages: 889-894, ISSN: 0027-8424
The Southern Ocean is a key region for the overturning and mixing of water masses within the global ocean circulation system. Because Southern Ocean dynamics are influenced by the Southern Hemisphere westerly winds (SWW), changes in the westerly wind forcing could significantly affect the circulation and mixing of water masses in this important location. While changes in SWW forcing during the Holocene (i.e., the last ∼11,700 y) have been documented, evidence of the oceanic response to these changes is equivocal. Here we use the neodymium (Nd) isotopic composition of absolute-dated cold-water coral skeletons to show that there have been distinct changes in the chemistry of the Southern Ocean water column during the Holocene. Our results reveal a pronounced Middle Holocene excursion (peaking ∼7,000–6,000 y before present), at the depth level presently occupied by Upper Circumpolar Deep Water (UCDW), toward Nd isotope values more typical of Pacific waters. We suggest that poleward-reduced SWW forcing during the Middle Holocene led to both reduced Southern Ocean deep mixing and enhanced influx of Pacific Deep Water into UCDW, inducing a water mass structure that was significantly different from today. Poleward SWW intensification during the Late Holocene could then have reinforced deep mixing along and across density surfaces, thus enhancing the release of accumulated CO2 to the atmosphere.
Klages JP, Salzmann U, Bickert T, et al., Temperate rainforests near the South Pole during peak Cretaceous warmth, Nature, ISSN: 0028-0836
The mid -Cretaceous was one of the warmest intervals of the past 140 million years31(Myr)1–5 driven by atmospheric CO2 levels around 1000 ppmv6. In the near absence of proximal geological records from south of the Antarctic Circle, it remains disputed whether polar ice could exist under such environmental conditions. Here we present results from a unique sedimentary sequence recovered from the West Antarctic shelf. This by far southernmost Cretaceous record contains an intact ~3 m-long network of in-situ fossil roots. The roots are embedded in a mudstone matrix bearing diverse pollen and spores, indicative of a temperate lowland rainforest environment at a palaeolatitude of ~82°S during the Turonian–Santonian (92–83 M yr). A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric CO2 contents of 1120–1680 ppmv and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo in high-CO2 climate world
Struve T, Wilson DJ, van de Flierdt T, et al., Mid-holocene expansion of Pacific deep water into the Southern Ocean, Proceedings of the National Academy of Sciences of USA, ISSN: 0027-8424
The Southern Ocean is a key region for the overturning and mixing of water masses within the global ocean circulation system. Because Southern Ocean dynamics are influenced by the Southern Hemisphere westerly winds (SWW), changes in the westerly wind forcing could significantly affect the circulation and mixing of water masses in this important location. While changes in SWW forcing during the Holocene (i.e. the last ~11,700 years) have been documented, evidence of the oceanic response to these changes is equivocal. Here we use the neodymium (Nd) isotopic composition of absolute-dated cold-water coral skeletons to show that there have been distinct changes in the chemistry of the Southern Ocean water column during the Holocene. Our results reveal a pronounced mid-Holocene excursion (peaking around ~7000 to 6000 years before present), at the depth level presently occupied by Upper Circumpolar Deep Water (UCDW), towards Nd isotope values more typical of Pacific waters. We suggest that poleward-reduced SWW forcing during the mid-Holocene led to both reduced Southern Ocean deep mixing and enhanced influx of Pacific Deep Water into UCDW, inducing a water mass structure that was significantly different from today. Poleward SWW intensification during the late Holocene could then have reinforced deep mixing along and across density surfaces, thus enhancing the release of accumulated CO2 to the atmosphere.
Blaser P, Frank M, van de Flierdt T, 2019, Revealing past ocean circulation with neodymium isotopes, Past Global Changes Magazine, Vol: 27, ISSN: 2411-605X
Xie RC, Rehkamper M, Grasse P, et al., 2019, Isotopic evidence for complex biogeochemical cycling of Cd in the eastern tropical South Pacific (vol 512, 134, 2019), EARTH AND PLANETARY SCIENCE LETTERS, Vol: 524, ISSN: 0012-821X
Pratt N, Chen T, Li T, et al., 2019, Temporal distribution and diversity of cold-water corals in the southwest Indian Ocean over the past 25,000 years, Deep Sea Research Part I: Oceanographic Research Papers, Vol: 149, ISSN: 0967-0637
Fossil cold-water corals can be used to reconstruct physical, chemical, and biological changes in the ocean because their skeleton often preserves ambient seawater signatures. Furthermore, patterns in the geographic and temporal extent of cold-water corals have changed through time in response to environmental conditions. Here we present taxonomic and dating results from a new collection of subfossil cold-water corals recovered from seamounts of the Southwest Indian Ocean Ridge. The area is a dynamic hydrographic region characterised by eastward flow of the Agulhas Return Current and the northernmost fronts of the Antarctic Circumpolar Current. In total, 122 solitary scleractinian corals and 27 samples of colonial scleractinian material were collected from water depths between 172 and 1395 m, corresponding to subtropical waters, Antarctic Intermediate Water (AAIW), and Upper Circumpolar Deep Water (UCDW). Fifteen species were identified, including eight species new to the region. The assemblage reflects the position of the seamounts in a transition zone between Indo-Pacific and Subantarctic biogeographic zones. Morphological variation in caryophyllids and the restriction of dendrophylliids to the southern seamounts could result from genetic isolation or reflect environmental conditions. Uranium-series dating using both rapid laser ablation and precise isotope dilution methods reveals their temporal distribution from the Last Glacial Maximum to the present day. Only one specimen of glacial age was found, while peaks in abundance occur around Heinrich Stadial 1 and the Younger Dryas, times at which ocean chemistry and food supply were likely to have presented optimal conditions for cold-water corals. A widespread regional preference of cold-water corals for UCDW over AAIW depths during the deglacial, the reverse of the modern situation, could be explained by higher dissolved oxygen concentrations and a temperature inversion that persisted into the early Holocene.
Xie RC, Rehkamper M, Grasse P, et al., 2019, Isotopic evidence for complex biogeochemical cycling of Cd in the eastern tropical South Pacific, Earth and Planetary Science Letters, Vol: 512, Pages: 134-146, ISSN: 0012-821X
Over the past decades, observations have confirmed decreasing oxygen levels and shoaling of oxygen minimum zones (OMZs) in the tropical oceans. Such changes impact the biogeochemical cycling of micronutrients such as Cd, but the potential consequences are only poorly constrained. Here, we present seawater Cd concentrations and isotope compositions for 12 depth profiles at coastal, nearshore and offshore stations from 4ºS to 14ºS in the eastern tropical South Pacific, where one of the world’s strongest OMZs prevails.The depth profiles of Cd isotopes display high δ114/110 Cd at the surface and decreasing δ114/110 Cd with increasing water depth, consistent with preferential utilization of lighter Cd isotopes during biological uptake in the euphotic zone and subsequent remineralization of the sinking biomass. In the surface and subsurface ocean, seawater displays similar δ114/110 Cd signatures of 0.47 ±0.23‰ to 0.82±0.05‰ across the entire eastern tropical South Pacific despite highly variable Cd concentrations between 0.01 and 0.84 nmol/kg. This observation, best explained by an open system steady-state fractionation model, contrasts with previous studies of the South Atlantic and South Pacific Oceans, where only Cd-deficient waters have a relatively constant Cd isotope signature. For the subsurface to about 500 m depth, the variability of seawater Cd isotope compositions can be modeled by mixing of remineralized Cd with subsurface water from the base of the mixed layer. In the intermediate and deep eastern tropical South Pacific (>500 m), seawater [Cd] and δ114/110 Cd appear to follow the distribution and mixing of major water masses. We identified modified AAIW of the ETSP to be more enriched in [Cd] than AAIW from the source region, whilst both water masses have similar δ114/110 Cd. A mass balance estimate thus constrains a δ114/110 Cd of between 0.38‰ and 0.56‰ for the
Dziadek R, Gohl K, Kaul N, et al., 2019, Elevated geothermal surface heat flow in the Amundsen Sea Embayment, West Antarctica, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 506, Pages: 530-539, ISSN: 0012-821X
Chase Z, Ellwood MJ, van de Flierdt T, 2018, Discovering the ocean's past through geochemistry, Elements, Vol: 14, Pages: 397-402, ISSN: 1811-5209
Trace elements and isotopes underlie many of the proxies used to reconstruct past ocean conditions. These proxies, recorded in diverse archives, are used to reconstruct seawater properties such as temperature, pH, and oxygen, or oceanic processes such as circulation, nutrient uptake, and biological productivity. Proxy calibration and validation requires a combination of ocean sediment core-top measurements, sediment trap studies, and laboratory- or field-based observations. New measurements of proxies in the modern ocean are rapidly illuminating the scope and limitations of each proxy while also helping to identify and evaluate new geochemical proxies that are based on trace elements and their isotopes.
Lambelet M, van de Flierdt T, Butler ECV, et al., 2018, The neodymium isotope fingerprint of Adélie coast bottom water, Geophysical Research Letters, Vol: 45, Pages: 11247-11256, ISSN: 0094-8276
Adélie Land Bottom Water (ALBW), a variety of Antarctic Bottom Water formed off the Adélie Land coast of East Antarctica, ventilates the abyssal layers of the Australian sector of the Southern Ocean as well as the eastern Indian and Pacific Oceans. We present the first dissolved neodymium (Nd) isotope and concentration measurements for ALBW. The summertime signature of ALBW is characterized by εNd = −8.9, distinct from Ross Sea Bottom Water, and similar to Weddell Sea Bottom Water. Adélie Land Shelf Water, the precursor water mass for wintertime ALBW, features the least radiogenic Nd fingerprint observed around Antarctica to date (εNd = −9.9). Local geology around Antarctica is important in setting the chemical signature of individual varieties of Antarctic Bottom Water, evident from the shelf water signature, which should be considered in the absence of direct wintertime observations.
McKay R, Exon N, Mueller D, et al., 2018, Developing community-based scientific priorities and new drilling proposals in the southern Indian and southwestern Pacific oceans, Scientific Drilling, Vol: 24, Pages: 61-70, ISSN: 1816-8957
An International Ocean Discovery Program (IODP) workshop was held at Sydney University, Australia, from 13 to 16 June 2017 and was attended by 97 scientists from 12 countries. The aim of the workshop was to investigate future drilling opportunities in the eastern Indian Ocean, southwestern Pacific Ocean, and the Indian and Pacific sectors of the Southern Ocean. The overlying regional sedimentary strata are underexplored relative to their Northern Hemisphere counterparts, and thus the role of the Southern Hemisphere in past global environmental change is poorly constrained. A total of 23 proposal ideas were discussed, with ∼ 12 of these deemed mature enough for active proposal development or awaiting scheduled site survey cruises. Of the remaining 11 proposals, key regions were identified where fundamental hypotheses are testable by drilling, but either site surveys are required or hypotheses need further development. Refinements are anticipated based upon regional IODP drilling in 2017/2018, analysis of recently collected site survey data, and the development of site survey proposals. We hope and expect that this workshop will lead to a new phase of scientific ocean drilling in the Australasian region in the early 2020s.
Pant NC, Jimenez-Espejo FJ, Cook CP, et al., 2018, Suspected meteorite fragments in marine sediments from East Antarctica, ANTARCTIC SCIENCE, Vol: 30, Pages: 307-321, ISSN: 0954-1020
Wilson DJ, Bertram R, Needham E, et al., 2018, Ice loss from the East Antarctic Ice Sheet during late Pleistocene interglacials, Nature, Vol: 561, Pages: 383-386, ISSN: 0028-0836
Understanding ice sheet behaviour in the geological past is essential for evaluating the role of the cryosphere in the climate system and for projecting rates and magnitudes of sea level rise in future warming scenarios1,2,3,4. Although both geological data5,6,7 and ice sheet models3,8 indicate that marine-based sectors of the East Antarctic Ice Sheet were unstable during Pliocene warm intervals, the ice sheet dynamics during late Pleistocene interglacial intervals are highly uncertain3,9,10. Here we provide evidence from marine sedimentological and geochemical records for ice margin retreat or thinning in the vicinity of the Wilkes Subglacial Basin of East Antarctica during warm late Pleistocene interglacial intervals. The most extreme changes in sediment provenance, recording changes in the locus of glacial erosion, occurred during marine isotope stages 5, 9, and 11, when Antarctic air temperatures11 were at least two degrees Celsius warmer than pre-industrial temperatures for 2,500 years or more. Hence, our study indicates a close link between extended Antarctic warmth and ice loss from the Wilkes Subglacial Basin, providing ice-proximal data to support a contribution to sea level from a reduced East Antarctic Ice Sheet during warm interglacial intervals. While the behaviour of other regions of the East Antarctic Ice Sheet remains to be assessed, it appears that modest future warming may be sufficient to cause ice loss from the Wilkes Subglacial Basin.
Khondoker R, Weiss DJ, van de Flierdt T, et al., 2018, New constraints on elemental and Pb and Nd isotope compositions of South American and Southern African aerosol sources to the South Atlantic Ocean, Chemie der Erde / Geochemistry, Vol: 78, Pages: 372-384, ISSN: 0009-2819
Improving the geochemical database available for characterising potential natural and anthropogenic aerosol sources from South America and Southern Africa is a critical precondition for studies aimed at understanding trace metal controls on the marine biogeochemical cycles of the South Atlantic Ocean. We here present new elemental and isotopic data for a wide range of sample types from South America and Southern Africa that are potentially important aerosol sources. This includes road dust from Buenos Aires and lichen samples from Johannesburg, soil dust from Patagonia, volcanic ash from the Andean volcanic belt, and aerosol samples from São Paulo. All samples were investigated for major (Al, Ca, Fe, Mg, Na, K, Mn) and trace element (Cd, Co, Cr, Cu, Ni, Pb, REE, Sc, Th, Y, V, Zn) concentrations and Nd and Pb isotopic compositions. We show that diagrams of 208Pb/207Pb vs. εNd, 208Pb/207Pb vs. Pb/Al, 1/[Pb], Zn/Al, Cd/Al, Cu/Al, and εNd vs. Pb/Al, and 1/[Nd] are best suited to separate South American and South African source regions as well as natural and anthropogenic sources. A subset of samples from Patagonia and the Andes was additionally subjected to separation of a fine (<5 μm) fraction and compared to the composition of the bulk sample. We show that differences in the geochemical signature of bulk samples between individual regions and source types are significantly larger than between grain sizes. Jointly, these findings present an important step forward towards a quantitative assessment of aeolian trace metal inputs to the South Atlantic Ocean.
The GEOTRACES Intermediate Data Product 2017 (IDP2017) is the second publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2016. The IDP2017 includes data from the Atlantic, Pacific, Arctic, Southern and Indian oceans, with about twice the data volume of the previous IDP2014. For the first time, the IDP2017 contains data for a large suite of biogeochemical parameters as well as aerosol and rain data characterising atmospheric trace element and isotope (TEI) sources. The TEI data in the IDP2017 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at crossover stations. The IDP2017 consists of two parts: (1) a compilation of digital data for more than 450 TEIs as well as standard hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing an on-line atlas that includes more than 590 section plots and 130 animated 3D scenes. The digital data are provided in several formats, including ASCII, Excel spreadsheet, netCDF, and Ocean Data View collection. Users can download the full data packages or make their own custom selections with a new on-line data extraction service. In addition to the actual data values, the IDP2017 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering and for statistical analysis. Metadata about data originators, analytical methods and original publications related to the data are linked in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2017 as section plots and rotating 3D scenes. The basin-wide 3D scenes combine data from many cruises and provide quick overviews of large-scale tracer distributions. These 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of tracer plumes near ocean margins or alo
Bertram RA, Wilson DJ, van de Flierdt T, et al., 2018, Pliocene deglacial event timelines and the biogeochemical response offshore Wilkes Subglacial Basin, East Antarctica, Earth and Planetary Science Letters, Vol: 494, Pages: 109-116, ISSN: 0012-821X
Significantly reduced ice coverage in Greenland and West Antarctica during the warmer-than-present Pliocene could account for ~10m of global mean sea level rise. Any sea level increase beyond this wouldrequire contributions from the East Antarctic Ice Sheet (EAIS). Previous studies have presented low-resolution geochemical evidence from the geological record, suggesting repeated ice advance and retreat inlow-lying areas of the EAIS such as the Wilkes Subglacial Basin. However, the rates and mechanisms of retreat events are less well constrained. Here we present orbitally-resolved marine detrital sediment provenance data, paired with ice-rafted debris and productivity proxies, during three time intervals from the middle to latePliocene at IODP Site U1361A, offshore of the Wilkes Subglacial Basin. Our new data reveal that Pliocene shifts in sediment provenance were paralleled by increases in marine productivity, while the onset of such changes was marked by peaks in ice-rafted debris mass accumulation rates. The coincidence of sediment provenance and marine productivity change argues against a switch in sediment delivery between ice streams, and instead suggests that deglacial warming triggered increased rates of iceberg calving, followed byinland retreat of the ice margin. Timescales from the onset of deglaciation to an inland retreated ice margin within the Wilkes Subglacial Basin are on the order of several thousand years. This geological evidence corroborates retreat rates determined from ice sheet modelling, and a contribution of ~3 to 4m of equivalent sea level rise from one of the most vulnerable areas of the East Antarctic Ice Sheet during interglacial intervals throughout the middleto late Pliocene.
Simoes Pereira P, van de Flierdt T, Hemming SR, et al., 2018, Geochemical fingerprints of glacially eroded bedrock from West Antarctica: detrital thermochronology, radiogenic isotope systematics and trace element geochemistry in Late Holocene glacial-marine sediments., Earth-Science Reviews, Vol: 182, Pages: 204-232, ISSN: 0012-8252
Geochemical provenance studies of glacial-marine sediments provide a powerful approach to describe subglacial geology, sediment transport pathways, and past ice sheet dynamics. The marine-based West Antarctic Ice Sheet (WAIS) is considered highly vulnerable to ocean warming and sea level rise that is likely to cause its rapid and irreversible retreat. Studies of its past response to climate change are hence essential for projecting its future behaviour. The application of radiogenic and trace element provenance studies for past ice sheet reconstructions requires surveying the geographic variability of geochemical compositions of glaciomarine sediments. In this study, we characterize the provenance of the detrital fraction of 67 Late Holocene marine sediment samples collected off the Pacific margin of West Antarctica (60°W to 160°W), including 40Ar/39Ar ages of individual hornblende and biotite grains (>150 μm), as well as Sr and Nd isotope and trace element composition of the fine-grained (<63 μm) sediment fraction. Overall, this approach allows differentiating West Antarctica into five source regions: the Antarctic Peninsula, Bellingshausen Sea, Amundsen Sea, Wrigley Gulf-Hobbs Coast and Sulzberger Bay. Minor geochemical variability is found within each individual sector due to local variability in onland geology. 40Ar/39Ar ages of iceberg-rafted hornblende and biotite grains record primarily Carboniferous to Lates Quaternary ages (~0 to 380 Ma), with a notable age peak of ~100 Ma, associated with plutonic intrusions or deformation events during the mid-Cretaceous. Permian-Jurassic 40Ar/39Ar ages are widespread in the Amundsen Sea sector, marking episodes of large-volume magmatism along the long-lived continental margin. Metasedimentary rocks and Late Cenozoic alkali basalts in West Antarctica cannot be detected using detrital hornblende and biotite 40Ar/39Ar ages due to the absence or small grain-size (i.e. <150 μm) of these minerals in s
McKay RM, De Santis L, Kulhanek DK, et al., 2018, International ocean discovery program expedition 374 preliminary report: Ross sea west antarctic ice sheet history ocean-ice sheet interactions and west antarctic ice sheet vulnerability: Clues from the neogene and quaternary record of the outer ross sea continental margin, Integrated Ocean Drilling Program: Preliminary Reports, Vol: 374, Pages: 1-63, ISSN: 1932-9423
The marine-based West Antarctic Ice Sheet (WAIS) is currently retreating due to shifting wind-driven oceanic currents that transport warm waters toward the ice margin, resulting in ice shelf thinning and accelerated mass loss of the WAIS. Previous results from geologic drilling on Antarctica's continental margins show significant variability in marine-based ice sheet extent during the late Neogene and Quaternary. Numerical models indicate a fundamental role for oceanic heat in controlling this variability over at least the past 20 My. Although evidence for past ice sheet variability has been collected in marginal settings, sedimentologic sequences from the outer continental shelf are required to evaluate the extent of past ice sheet variability and the associated oceanic forcings and feedbacks. International Ocean Discovery Program Expedition 374 drilled a latitudinal and depth transect of five drill sites from the outer continental shelf to rise in the eastern Ross Sea to resolve the relationship between climatic and oceanic change and WAIS evolution through the Neogene and Quaternary. This location was selected because numerical ice sheet models indicate that this sector of Antarctica is highly sensitive to changes in ocean heat flux. The expedition was designed for optimal data-model integration and will enable an improved understanding of the sensitivity of Antarctic Ice Sheet (AIS) mass balance during warmer-than-present climates (e.g., the Pleistocene "super interglacials," the mid-Pliocene, and the late early to middle Miocene). The principal goals of Expedition 374 were to • Evaluate the contribution of West Antarctica to far-field ice volume and sea level estimates; • Reconstruct ice-proximal atmospheric and oceanic temperatures to identify past polar amplification and assess its forcings and feedbacks; • Assess the role of oceanic forcing (e.g., sea level and temperature) on AIS stability/instability; • Identify the sensitivit
Coxall HK, Huck CE, Huber M, et al., 2018, Export of nutrient rich Northern Component Water preceded early Oligocene Antarctic glaciation, Nature Geoscience, Vol: 11, Pages: 190-196, ISSN: 1752-0894
The onset of the North Atlantic Deep Water formation is thought to have coincided with Antarctic ice-sheet growth about 34 million years ago (Ma). However, this timing is debated, in part due to questions over the geochemical signature of the ancient Northern Component Water (NCW) formed in the deep North Atlantic. Here we present detailed geochemical records from North Atlantic sediment cores located close to sites of deep-water formation. We find that prior to 36 Ma, the northwestern Atlantic was stratified, with nutrient-rich, low-salinity bottom waters. This restricted basin transitioned into a conduit for NCW that began flowing southwards approximately one million years before the initial Antarctic glaciation. The probable trigger was tectonic adjustments in subarctic seas that enabled an increased exchange across the Greenland–Scotland Ridge. The increasing surface salinity and density strengthened the production of NCW. The late Eocene deep-water mass differed in its carbon isotopic signature from modern values as a result of the leakage of fossil carbon from the Arctic Ocean. Export of this nutrient-laden water provided a transient pulse of CO2 to the Earth system, which perhaps caused short-term warming, whereas the long-term effect of enhanced NCW formation was a greater northward heat transport that cooled Antarctica.
Bridgestock L, Rehkamper M, van de Flierdt T, et al., 2018, The distribution of lead concentrations and isotope compositions in the eastern Tropical Atlantic Ocean, Geochimica et Cosmochimica Acta, Vol: 225, Pages: 36-51, ISSN: 0016-7037
Anthropogenic emissions have dominated marine Pb sources during the past century. Here we present Pb concentrations and isotope compositions for ocean depth profiles collected in the eastern Tropical Atlantic Ocean (GEOTRACES section GA06), to trace the transfer of anthropogenic Pb into the ocean interior. Variations in Pb concentration and isotope composition were associated with changes in hydrography. Water masses ventilated in the southern hemisphere generally featured lower 206Pb/207Pb and 208Pb/207Pb ratios than those ventilated in the northern hemisphere, in accordance with Pb isotope data of historic anthropogenic Pb emissions. The distributions of Pb concentrations and isotope compositions in northern sourced waters were consistent with differences in their ventilation timescales. For example, a Pb concentration maximum at intermediate depth (600–900 m, 35 pmol kg−1) in waters sourced from the Irminger/Labrador Seas, is associated with Pb isotope compositions (206Pb/207Pb = 1.1818–1.1824, 208Pb/207Pb = 2.4472–2.4483) indicative of northern hemispheric emissions during the 1950s and 1960s close to peak leaded petrol usage, and a transit time of ∼50–60 years. In contrast, North Atlantic Deep Water (2000–4000 m water depth) featured lower Pb concentrations and isotope compositions (206Pb/207Pb = 1.1762–1.184, 208Pb/207Pb = 2.4482–2.4545) indicative of northern hemispheric emissions during the 1910s and 1930s and a transit time of ∼80–100 years. This supports the notion that transient anthropogenic Pb inputs are predominantly transferred into the ocean interior by water mass transport. However, the interpretation of Pb concentration and isotope composition distributions in terms of ventilation timescales and pathways is complicated by (1) the chemical reactivity of Pb in the ocean, and (2) mixing of waters ventilated during different time periods. The complex effects of water mass mixing on Pb distri
Sangiorgi F, Bijl PK, Passchier S, et al., 2018, Southern Ocean warming and Wilkes Land ice sheet retreat during the mid Miocene, Nature Communications, Vol: 9, ISSN: 2041-1723
Observations and model experiments highlight the importance of ocean heat in forcing icesheet retreat during the present and geological past, but past ocean temperature data arevirtually missing in ice sheet proximal locations. Here we document paleoceanographicconditions and the (in)stability of the Wilkes Land subglacial basin (East Antarctica) duringthe mid-Miocene (~17–13.4 million years ago) by studying sediment cores from offshoreAdélie Coast. Inland retreat of the ice sheet, temperate vegetation, and warm oligotrophicwaters characterise the mid-Miocene Climatic Optimum (MCO; 17–14.8 Ma). After the MCO,expansion of a marine-based ice sheet occurs, but remains sensitive to melting upon episodicwarm water incursions. Our results suggest that the mid-Miocene latitudinal temperaturegradient across the Southern Ocean never resembled that of the present day. We demonstratethat a strong coupling of oceanic climate and Antarctic continental conditions existedand that the East Antarctic subglacial basins were highly sensitive to ocean warming.
Gohl K, Freudenthal T, Hillenbrand C-D, et al., 2017, MeBo70 seabed drilling on a polar continental shelf: operationalreport and lessons from drilling in the Amundsen Sea Embaymentof West Antarctica, Geochemistry, Geophysics, Geosystems, Vol: 18, Pages: 4235-4250, ISSN: 1525-2027
A multi-barrel seabed drill rig was used for the first time to drill unconsolidated sediments and consolidated sedimentary rocks from an Antarctic shelf with core recoveries between 7 and 76%. We deployed the MARUM-MeBo70 drill device at nine drill sites in the Amundsen Sea Embayment. Three sites were located on the inner shelf of Pine Island Bay from which soft sediments, presumably deposited at high sedimentation rates in isolated small basins, were recovered from drill depths of up to 36 m below seafloor. Six sites were located on the middle shelf of the eastern and western embayment. Drilling at five of these sites recovered consolidated sediments and sedimentary rocks from dipping strata spanning ages from Late Cretaceous to Miocene. This report describes the initial coring results, the challenges posed by drifting icebergs and sea ice, and technical issues related to deployment of the MeBo70. We also present recommendations for similar future drilling campaigns on polar continental shelves.
Pierce EL, van de Flierdt T, Williams T, et al., 2017, Evidence for a dynamic East Antarctic ice sheet during the mid-Miocene climate transition, Earth and Planetary Science Letters, Vol: 478, Pages: 1-13, ISSN: 0012-821X
The East Antarctic ice sheet underwent a major expansion during the Mid-Miocene Climate Transition, around 14 Ma, lowering sea level by ∼60 m. However, direct or indirect evidence of where changes in the ice sheet occurred is limited. Here we present new insights on timing and locations of ice sheet change from two drill sites offshore East Antarctica. IODP Site U1356, Wilkes Land, and ODP Site 1165, Prydz Bay are located adjacent to two major ice drainage areas, the Wilkes Subglacial Basin and the Lambert Graben. Ice-rafted detritus (IRD), including dropstones, was deposited in concentrations far exceeding those known in the rest of the Miocene succession at both sites between 14.1 and 13.8 Ma, indicating that large amounts of IRD-bearing icebergs were calved from independent drainage basins during this relatively short interval. At Site U1356, the IRD was delivered in distinct pulses, suggesting that the overall ice advance was punctuated by short periods of ice retreat in the Wilkes Subglacial Basin. Provenance analysis of the mid-Miocene IRD and fine-grained sediments provides additional insights on the movement of the ice margin and subglacial geology. At Site U1356, the dominant 40Ar/39Ar thermochronological age of the ice-rafted hornblende grains is 1400–1550 Ma, differing from the majority of recent IRD in the area, from which we infer an inland source area of this thermochronological age extending along the eastern part of the Adélie Craton, which forms the western side of the Wilkes Subglacial Basin. Neodymium isotopic compositions from the terrigenous fine fraction at Site U1356 imply that the ice margin periodically expanded from high ground well into the Wilkes Subglacial Basin during periods of MMCT ice growth. At Site 1165, MMCT pebble-sized IRD are sourced from both the local Lambert Graben and the distant Aurora Subglacial Basin drainage area. Together, the occurrence and provenance of the IRD and glacially-eroded sediment at these
Huck CE, van de Flierdt T, Bohaty SM, et al., 2017, Antarctic climate, Southern Ocean circulation patterns, and deep-water formation during the Eocene, Paleoceanography, Vol: 32, ISSN: 1944-9186
We assess early-to-middle Eocene seawater neodymium (Nd) isotope records from seven Southern Ocean deep-sea drill sites to evaluate the role of Southern Ocean circulation in long-term Cenozoic climate change. Our study sites are strategically located on either side of the Tasman Gateway and are positioned at a range of shallow (<500 m) to intermediate/deep (~1000–2500 m) paleowater depths. Unradiogenic seawater Nd isotopic compositions, reconstructed from fish teeth at intermediate/deep Indian Ocean pelagic sites (Ocean Drilling Program (ODP) Sites 738 and 757 and Deep Sea Drilling Project (DSDP) Site 264), indicate a dominant Southern Ocean-sourced contribution to regional deep waters (εNd(t) = −9.3 ± 1.5). IODP Site U1356 off the coast of Adélie Land, a locus of modern-day Antarctic Bottom Water production, is identified as a site of persistent deep water formation from the early Eocene to the Oligocene. East of the Tasman Gateway an additional local source of intermediate/deep water formation is inferred at ODP Site 277 in the SW Pacific Ocean (εNd(t) = −8.7 ± 1.5). Antarctic-proximal shelf sites (ODP Site 1171 and Site U1356) reveal a pronounced erosional event between 49 and 48 Ma, manifested by ~2 εNd unit negative excursions in seawater chemistry toward the composition of bulk sediments at these sites. This erosional event coincides with the termination of peak global warmth following the Early Eocene Climatic Optimum and is associated with documented cooling across the study region and increased export of Antarctic deep waters, highlighting the complexity and importance of Southern Ocean circulation in the greenhouse climate of the Eocene.
Cook CP, Hemming SR, van de Flierdt T, et al., 2017, Glacial erosion of East Antarctica in the Pliocene: a comparative study of multiple marine sediment provenance tracers, Chemical Geology, Vol: 466, Pages: 199-218, ISSN: 1872-6836
The history of the East Antarctic ice sheet provides important understanding of its potential future behaviour in a warming world. The provenance of glaciomarine sediments can provide insights into this history, if the underlying continent eroded by the ice sheet is made of distinct geological terranes that can be distinguished by the mineralogy, petrology and/or geochemistry of the eroded sediment. We here present a multi-proxy provenance investigation on Pliocene sediments from Integrated Ocean Drilling Program (IODP) Site U1361, located offshore of the Wilkes Subglacial Basin, East Antarctica. We compare Nd and Sr isotopic compositions of < 63 μm detrital fractions, clay mineralogy of < 2 μm fractions, 40Ar/39Ar ages of > 150 μm ice-rafted hornblende grains, and petrography of > 2 mm ice-rafted clasts and > 150 μm mineral grains. Pliocene fine-grained marine sediments have Nd and Sr isotopic compositions, clay mineralogy, and clast characteristics that can be explained by mixing of sediments eroded from predominantly proximal crystalline terranes with material derived from inland sources from within the currently glaciated Wilkes Subglacial Basin. Conversely, evidence for such an inland source is absent from ice-rafted hornblende ages. We render a lithological bias against hornblende grains in the doleritic and sedimentary units within the basin the most likely explanation for this observation. 40Ar/39Ar hornblende ages however record additional provenance from the distal margins of the Ross Sea, and possibly even the West Antarctic area of Marie Byrd Land. The latter lies > 2000 km to the east and hints at significant iceberg release from the West Antarctic ice sheet during warm intervals of the Pliocene. Together our results make a strong case for combining geochemical and mineralogical signatures of coarse- and fine-grained glaciomarine sediment fractions in order to derive robust provenance interpretations in ice covered areas.
Bridgestock L, Rehkamper M, van de Flierdt T, et al., 2017, The Cd isotope composition of atmospheric aerosols from the tropical Atlantic Ocean, Geophysical Research Letters, Vol: 44, Pages: 2932-2940, ISSN: 1944-8007
Stable isotope compositions can potentially be used to trace atmospheric Cd inputs to the surface ocean and anthropogenic Cd emissions to the atmosphere. Both of these applications may provide valuable insights into the effects of anthropogenic activities on the cycling of Cd in the environment. However, a lack of constraints for the Cd isotope compositions of atmospheric aerosols is currently hindering such studies. Here we present stable Cd isotope data for aerosols collected over the Tropical Atlantic Ocean. The samples feature variable proportions of mineral dust-derived and anthropogenic Cd, yet exhibit similar isotope compositions, thus negating the distinction of these Cd sources by using isotopic signatures in this region. Isotopic variability between these two atmospheric Cd sources may be identified in other areas, and thus warrants further investigation. Regardless, these data provide important initial constraints on the isotope composition of atmospheric Cd inputs to the ocean.
Deaney EL, Barker S, van de Flierdt T, 2017, Timing and nature of AMOC recovery across Termination 2 and magnitude of deglacial CO2 change, Nature Communications, Vol: 8, ISSN: 2041-1723
Large amplitude variations in atmospheric CO2were associated with glacial terminationsof the Late Pleistocene. Here we provide multiple lines of evidence suggesting that theB20 p.p.m.v. overshoot in CO2at the end of Termination 2 (T2)B129 ka was associated withan abrupt (r400 year) deepening of Atlantic Meridional Overturning Circulation (AMOC).In contrast to Termination 1 (T1), which was interrupted by the Bølling-Allerød (B-A), AMOCrecovery did not occur until the very end of T2, and was characterized by pronouncedformation of deep waters in the NW Atlantic. Considering the variable influences of oceancirculation change on atmospheric CO2, we suggest that the net change in CO2across thelast 2 terminations was approximately equal if the transient effects of deglacial oscillations inocean circulation are taken into account.
Wilson DJ, van de Flierdt T, Adkins JF, 2017, Lead isotopes in deep-sea coral skeletons: ground-truthingand a first deglacial Southern Ocean record, Geochimica et Cosmochimica Acta, Vol: 204, Pages: 350-374, ISSN: 0016-7037
Past changes in seawater lead (Pb) isotopes record the temporal evolution of anthropogenic pollution, continental weathering inputs, and ocean current transport. To advance our ability to reconstruct this signature, we present methodological developments that allow us to make precise and accurate Pb isotope measurements on deep-sea coral aragonite, and apply our approach to generate the first Pb isotope record for the glacial to deglacial mid-depth Southern Ocean. Our refined methodology includes a two-step anion exchange chemistry procedure and measurement using a 207Pb-204Pb double spike on a ThermoFinnigan Triton TIMS instrument. By employing a 1012Ω resistor (in place of a 10Ω resistor) to measure the low-abundance 204Pb ion beam, we improve the internal precision on 206,207,208Pb/204Pb for a 2 ng load of NIST-SRM-981 Pb from typically ~420 ppm to ~260 ppm (2 s.e.), and the long term external reproducibility from ~960 ppm to ~580 ppm (2 s.d.). Furthermore, for a typical 500 mg coral sample with low Pb concentrations (~6-10 ppb yielding ~3-5 ng Pb for analysis), we obtain a comparable internal precision of ~150-250 ppm for 206,207,208Pb/204Pb, indicating a good sensitivity for tracing natural Pb sources to the oceans. Successful extraction of a seawater signal from deep-sea coral aragonite further relies on careful physical and chemical cleaning steps, which are necessary to remove anthropogenic Pb contaminants and obtain results that are consistent with ferromanganese crusts. Applying our approach to a collection of late glacial and deglacial corals (~12-40 ka BP) from south of Tasmania at ~1.4-1.7 km water depth, we generated the first intermediate water Pb isotope record from the Southern Ocean. That record reveals millennial timescale variability, controlled by binary mixing between two Pb sources, but no distinct glacial-inter
Struve T, van de Flierdt T, Burke A, et al., 2017, Neodymium isotopes and concentrations in aragonitic scleractinian cold-water coral skeletons - Modern calibration and evaluation of palaeo-applications, Chemical Geology, Vol: 453, Pages: 146-168, ISSN: 1872-6836
Cold-water corals (CWCs) are unique archives of mid-depth ocean chemistry and have been used successfully to reconstruct the neodymium (Nd) isotopic composition of seawater from a number of species. High and variable Nd concentrations in fossil corals however pose the question as to how Nd is incorporated into their skeletons.We here present new results on modern specimens of Desmophyllum dianthus, Balanophyllia malouinensis, and Flabellum curvatum, collected from the Drake Passage, and Madrepora oculata, collected from the North Atlantic. All modern individuals were either collected alive or uranium-series dated to be < 500 years old for comparison with local surface sediments and seawater profiles. Modern coral Nd isotopic compositions generally agree with ambient seawater values, which in turn are consistent with previously published seawater analyses, supporting small vertical and lateral Nd isotope gradients in modern Drake Passage waters. Two Balanophyllia malouinensis specimens collected live however deviate by up to 0.6 epsilon units from ambient seawater. We therefore recommend that this species should be treated with caution for the reconstruction of past seawater Nd isotopic compositions.Seventy fossil Drake Passage CWCs were furthermore analysed for their Nd concentrations, revealing a large range from 7.3 to 964.5 ng/g. Samples of the species D. dianthus and Caryophyllia spp. show minor covariation of Nd with 232Th content, utilised to monitor contaminant phases in cleaned coral aragonite. Strong covariations between Nd and Th concentrations are however observed in the species B. malouinensis and G. antarctica. In order to better constrain the source and nature of Nd in the cleaned aragonitic skeletons, a subset of sixteen corals was investigated for its rare earth element (REE) content, as well as major and trace element geochemistry. Our new data provide supporting evidence that the applied cleaning protocol efficiently removes contaminant lithogen
van de Flierdt T, Griffiths AM, Lambelet M, et al., 2016, Neodymium in the oceans: a global database, a regional comparison and implications for palaeoceanographic research, Journal: Philosophical Transactions A: Mathematical, Physical and Engineering Sciences, Vol: 374, ISSN: 1471-2962
The neodymium (Nd) isotopic composition of seawater has been used extensively to reconstruct ocean circulation on a variety of time scales. However, dissolved neodymium concentrations and isotopes do not always behave conservatively, and quantitative deconvolution of this non-conservative component can be used to detect trace metal inputs and isotopic exchange at ocean–sediment interfaces. In order to facilitate such comparisons for historical datasets, we here provide an extended global database for Nd isotopes and concentrations in the context of hydrography and nutrients. Since 2010, combined datasets for a large range of trace elements and isotopes are collected on international GEOTRACES section cruises, alongside classical nutrient and hydrography measurements. Here, we take a first step towards exploiting these datasets by comparing high-resolution Nd sections for the western and eastern North Atlantic in the context of hydrography, nutrients and aluminium (Al) concentrations. Evaluating those data in tracer–tracer space reveals that North Atlantic seawater Nd isotopes and concentrations generally follow the patterns of advection, as do Al concentrations. Deviations from water mass mixing are observed locally, associated with the addition or removal of trace metals in benthic nepheloid layers, exchange with ocean margins (i.e. boundary exchange) and/or exchange with particulate phases (i.e. reversible scavenging). We emphasize that the complexity of some of the new datasets cautions against a quantitative interpretation of individual palaeo Nd isotope records, and indicates the importance of spatial reconstructions for a more balanced approach to deciphering past ocean changes.
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.