Ocean mixing and ice-sheet control of seawater
            <sup>234</sup>
            U/
            <sup>238</sup>
            U during the last deglaciation

Chen, Tianyu; Robinson, Laura F.; Beasley, Matthew P.; Claxton, Louis M.; Andersen, Morten B.; Gregoire, Lauren; Wadham, Jemma L.; Fornari, Daniel J.; Harpp, K. S.

doi:10.1126/science.aag1015

Cited by 48 publications

(52 citation statements)

References 86 publications

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“…The higher ratios we observed are due to the high mobility of 234 U brought about from α recoil in the minerals of the rocks in contact with the brine subglacially, a process that is likely enhanced by glacial grinding and the resulting decrease in grain size, which increases the amount of 234 U in waters after α recoil (Robinson et al, ). Recent work has suggested that water released subglacially during the beginning of the last major deglaciation significantly increased the global ocean 234 U/ 238 U ratio (Chen et al, ). Our work certainly supports the notion that subglacial drainage can have very high 234 U/ 238 U values due to the increased residence time of the water in contact with subglacial materials that have been ground by glacier advance and retreat.…”

Section: Resultsmentioning

confidence: 99%

The Geochemistry of Englacial Brine From Taylor Glacier, Antarctica

et al. 2019

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Blood Falls is a hypersaline, iron‐rich discharge at the terminus of the Taylor Glacier in the McMurdo Dry Valleys, Antarctica. In November 2014, brine in a conduit within the glacier was penetrated and sampled using clean‐entry techniques and a thermoelectric melting probe called the IceMole. We analyzed the englacial brine sample for filterable iron (fFe), total Fe, major cations and anions, nutrients, organic carbon, and perchlorate. In addition, aliquots were analyzed for minor and trace elements and isotopes including δD and δ18O of water, δ34S and δ18O of sulfate, 234U, 238U, δ11B, 87Sr/86Sr, and δ81Br. These measurements were made in order to (1) determine the source and geochemical evolution of the brine and (2) compare the chemistry of the brine to that of nearby hypersaline lake waters and previous supraglacially sampled collections of Blood Falls outflow that were interpreted as end‐member brines. The englacial brine had higher Cl− concentrations than the Blood Falls end‐member outflow; however, other constituents were similar. The isotope data indicate that the water in the brine is derived from glacier melt. The H4SiO4 concentrations and U and Sr isotope suggest a high degree of chemical weathering products. The brine has a low N:P ratio of ~7.2 with most of the dissolved inorganic nitrogen in the form of NH4+. Dissolved organic carbon concentrations are similar to end‐member outflow values. Our results provide strong evidence that the original source of solutes in the brine was ancient seawater, which has been modified with the addition of chemical weathering products.

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Section: Resultsmentioning

confidence: 99%

The Geochemistry of Englacial Brine From Taylor Glacier, Antarctica

et al. 2019

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“…Pulses of detrital glacier flour and ice‐rafted debris (IRD) delivered to the high‐latitude North Atlantic during abrupt deglaciation meltwater events (Bond et al, ) may also have contributed to an enhanced rapid dissolved Si supply to the ocean that appeared “faster” than the “accepted” 10 ka residence time that silica would allow. This response may be similar to the transient deglacial subglacial‐meltwater uranium pulses reported to have driven rapid whole ocean changes in 234 U/ 238 U, much faster than the canonical 0.4 Ma residence time for U in today's ocean (Chen et al, ). This silica change could result from a global increase in Si supply (e.g., dust: (King et al, ; Kurtz et al, ; Mahowald et al, ); rivers: (Froelich et al, )) or a decrease in Si removal (e.g., biogenic silica burial: (Anderson et al, )).…”

Section: Discussionmentioning

confidence: 99%

Whole‐Ocean Changes in Silica and Ge/Si Ratios During the Last Deglacial Deduced From Long‐Lived Giant Glass Sponges

Jochum

Schuessler

Wang

et al. 2017

Geophysical Research Letters

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Silicon is a keystone nutrient in the ocean for understanding climate change because of the importance of Southern Ocean diatoms in taking up CO2 from the surface ocean‐atmosphere system and sequestering carbon into the deep sea. Here we report on silicon isotopes and germanium‐to‐silicon ratios in giant glass spicules of deep‐sea sponge Monorhaphis chuni over the past 17,000 years. In situ measurements of Si isotopes and Ge concentrations show systematic variations from rim to center of the cross sections. When calibrated against seawater concentrations using data from modern spicule rims, sponge data indicate that dissolved silica concentrations in the deep Pacific were ~12% higher during the early deglacial. These deep Pacific Ocean data help to fill an important global gap in paleo‐nutrient records. Either continental sources supplied more silica to the deglacial ocean and/or biogenic silica burial was lower, both of which may have affected atmospheric CO2.

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“…Instead, in support of our framework, the AMOC transition from warm on to off is likely a two-step process (Ng et al, 2018): in response to gradual cooling associated with atmospheric CO 2 decrease (Barker et al, 2015), an early slowdown of circulation may have occurred. It was accompanied by an early external freshwater input (Chen et al, 2016;Ménot et al, 2006) mostly likely from Eurasian ice sheets (Grousset et al, 2001;Ng et al, 2018;Peck et al, 2006). As a second step, an off state persisted for quite a few hundreds to a thousand years, with substantial ice rafting events from the LIS (Heinrich events) as a consequence (Barker et al, 2015;Hodell et al, 2017;Marcott et al, 2011).…”

Section: Externally Forced Millennial Variabilitymentioning

confidence: 99%

Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis

et al. 2019

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The notion that the Atlantic Meridional Overturning Circulation (AMOC) can have more than one stable equilibrium emerged in the 1980s as a powerful hypothesis to explain rapid climate variability during the Pleistocene. Ever since, the idea that a temporary perturbation of the AMOC—or a permanent change in its forcing—could trigger an irreversible collapse has remained a reason for concern. Here we review literature on the equilibrium stability of the AMOC and present a synthesis that puts our understanding of past and future AMOC behavior in a unifying framework. This framework is based on concepts from Dynamical Systems Theory, which has proven to be an important tool in interpreting a wide range of model behavior. We conclude that it cannot be ruled out that the AMOC in our current climate is in, or close to, a regime of multiple equilibria. But there is considerable uncertainty in the location of stability thresholds with respect to our current climate state, so we have no credible indications of where our present‐day AMOC is located with respect to thresholds. We conclude by identifying gaps in our knowledge and proposing possible ways forward to address these gaps.

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Ocean mixing and ice-sheet control of seawater ²³⁴ U/ ²³⁸ U during the last deglaciation

Abstract: Abstract:Seawater 234 U/ 238 U provides global-scale information on continental weathering and is vital for marine U-series geochronology. Previous evidence supports an increase in 234 U/ 238 U since the last glacial, but the timing and amplitude of its variability was poorly

Cited by 48 publications

References 86 publications

The Geochemistry of Englacial Brine From Taylor Glacier, Antarctica

The Geochemistry of Englacial Brine From Taylor Glacier, Antarctica

Whole‐Ocean Changes in Silica and Ge/Si Ratios During the Last Deglacial Deduced From Long‐Lived Giant Glass Sponges

Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis

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Ocean mixing and ice-sheet control of seawater 234 U/ 238 U during the last deglaciation

Abstract: Abstract:Seawater 234 U/ 238 U provides global-scale information on continental weathering and is vital for marine U-series geochronology. Previous evidence supports an increase in 234 U/ 238 U since the last glacial, but the timing and amplitude of its variability was poorly

Cited by 48 publications

References 86 publications

The Geochemistry of Englacial Brine From Taylor Glacier, Antarctica

The Geochemistry of Englacial Brine From Taylor Glacier, Antarctica

Whole‐Ocean Changes in Silica and Ge/Si Ratios During the Last Deglacial Deduced From Long‐Lived Giant Glass Sponges

Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis

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Product

Resources

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Ocean mixing and ice-sheet control of seawater ²³⁴ U/ ²³⁸ U during the last deglaciation