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  • 1
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    Ice volume and climate changes from a 6000 year sea-level record in French Polynesia (2018)
    Nature Research
    In:  Nature Communications, 9 (285).
    Details
    Publication Date: 2021-02-08
    Description: Mid- to late-Holocene sea-level records from low-latitude regions serve as an important baseline of natural variability in sea level and global ice volume prior to the Anthropocene. Here, we reconstruct a high-resolution sea-level curve encompassing the last 6000 years based on a comprehensive study of coral microatolls, which are sensitive low-tide recorders. Our curve is based on microatolls from several islands in a single region and comprises a total of 82 sea-level index points. Assuming thermosteric contributions are negligible on millennial time scales, our results constrain global ice melting to be 1.5–2.5 m (sea-level equivalent) since ~5500 years before present. The reconstructed curve includes isolated rapid events of several decimetres within a few centuries, one of which is most likely related to loss from the Antarctic ice sheet mass around 5000 years before present. In contrast, the occurrence of large and flat microatolls indicates periods of significant sea-level stability lasting up to ~300 years.
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  • 2
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    Impact of high CO2 on the geochemistry of the coralline algae Lithothamnion glaciale (2016)
    Nature Research
    In:  Scientific Reports, 6 (20572).
    Details
    Publication Date: 2019-02-01
    Description: Coralline algae are a significant component of the benthic ecosystem. Their ability to withstand physical stresses in high energy environments relies on their skeletal structure which is composed of high Mg-calcite. High Mg-calcite is, however, the most soluble form of calcium carbonate and therefore potentially vulnerable to the change in carbonate chemistry resulting from the absorption of anthropogenic CO2 by the ocean. We examine the geochemistry of the cold water coralline alga Lithothamnion glaciale grown under predicted future (year 2050) high pCO2 (589 μatm) using Electron microprobe and NanoSIMS analysis. In the natural and control material, higher Mg calcite forms clear concentric bands around the algal cells. As expected, summer growth has a higher Mg content compared to the winter growth. In contrast, under elevated CO2 no banding of Mg is recognisable and overall Mg concentrations are lower. This reduction in Mg in the carbonate undermines the accuracy of the Mg/Ca ratio as proxy for past temperatures in time intervals with significantly different carbonate chemistry. Fundamentally, the loss of Mg in the calcite may reduce elasticity thereby changing the structural properties, which may affect the ability of L. glaciale to efficiently function as a habitat former in the future ocean.
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  • 3
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    230Th-238U disequilibrium in East Scotia back-arc basalts: implications for slab contribution and flux rates (2003)
    GSA, Geological Society of America
    In:  Geology, 31 (8). pp. 693-696.
    Details
    Publication Date: 2017-09-26
    Description: New U-series isotope data for lavas from the East Scotia backarc spreading center span a large range in (230Th/232Th) and (238U/232Th). Most of the backarc lavas have (238U/230Th) 〈 1, similar to the composition of mid-oceanic-ridge basalts (MORB). Lavas from two segments have (238U/230Th) 〉 1 and are enriched in fluid-mobile elements, implying a recent addition of a U-rich slab-derived component. The data from one segment suggest an influence from an aqueous fluid from altered MORB, whereas samples from the other slab-influenced segment show addition of a sediment melt. The slab-influenced samples form a distinct trend in the equiline diagram between aqueous fluid and sediment melt that is suggested to be a mixing line rather than an isochron.
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  • 4
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    Internal pH regulation facilitates in situ long-term acclimation of massive corals to end-of-century carbon dioxide conditions (2016)
    Nature Research
    In:  Scientific Reports, 6 (Art. Nr. 30688).
    Details
    Publication Date: 2020-06-18
    Description: The resilience of tropical corals to ocean acidification depends on their ability to regulate the pH within their calcifying fluid (pHcf). Recent work suggests pHcf homeostasis under short-term exposure to pCO2 conditions predicted for 2100, but it is still unclear if pHcf homeostasis can be maintained throughout a corals lifetime. At CO2 seeps in Papua New Guinea, massive Porites corals have grown along a natural seawater pH gradient for decades. This natural gradient, ranging from pH 8.1–7.4, provides an ideal platform to determine corals’ pHcf (using boron isotopes). Porites maintained a similar pHcf (~8.24) at both a control (pH 8.1) and seep-influenced site (pH 7.9). Internal pHcf was slightly reduced (8.12) at seawater pH 7.6, and decreased to 7.94 at a site with a seawater pH of 7.4. A growth response model based on pHcf mirrors the observed distribution patterns of this species in the field. We suggest Porites has the capacity to acclimate after long-time exposure to end-of-century reduced seawater pH conditions and that strong control over pHcf represents a key mechanism to persist in future oceans. Only beyond end-of-century pCO2 conditions do they face their current physiological limit of pH homeostasis and pHcf begins to decrease.
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  • 5
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    Megatsunami deposits on Kohala volcano, Hawaii, from flank collapse of Mauna Loa (2004)
    GSA, Geological Society of America
    In:  Geology, 32 (9). pp. 741-744.
    Details
    Publication Date: 2017-08-07
    Description: The origin of coastal and high-elevation marine gravels on the Hawaiian islands of Lanai and Molokai is controversial, because the vertical tectonics of these islands is poorly constrained. The gravels are either from eustatic highstands or were left by massive tsunamis from offshore giant landslides. In contrast, at Kohala on the island of Hawaii, where continuous subsidence is well established, lithofacies analysis and dating of a fossiliferous marine conglomerate 1.5–61 m above present sea level support a tsunami origin and indicate a runup of 〉400 m 〉6 km inland. The conglomerate age, 110 ± 10 ka, suggests a tsunami caused by the ca. 120 ka giant Alika 2 landslide from nearby Mauna Loa volcano.
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  • 6
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    Uplift of Oahu, Hawaii, during the past 500 k.y. as recorded by elevated reef deposits (2011)
    GSA, Geological Society of America
    In:  Geology, 38 (1). pp. 27-30.
    Details
    Publication Date: 2017-05-18
    Description: Studies of paleo–sea level and past climate have focused upon proxy methods in ice and deep-sea cores and more direct information provided by past shorelines, in some cases preserved as raised or submerged reef deposits in tropical areas. Paleo-shorelines need to be constrained by accurate tectonic history because these environments and their marine deposits can be confused with past tsunami deposits and vice versa. A maximum 21-m-high extensive emerged reef on Oahu, Hawaii, U-series dated to 334 ± 17 ka, together with a mean U-series age of 335 ± 22 ka (n = 5) for slightly higher, energetic shoreline deposits nearby, suggest a marine isotope stage 9 (MIS 9) highstand, and extend the earlier work indicating a linear uplift for Oahu of 0.060 ± 0.001 mm/yr over the past 500 k.y. Five of the past six major emerged interglacial highstand reefs on Oahu have been identified, and these data provide little evidence for past maximum sea levels significantly greater than 2 m above the sea level datum at that time. There is currently no evidence for a MIS 11 highstand on Oahu.
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  • 7
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    Uplift of Oahu, Hawaii, during the past 500 k.y. as recorded by elevated reef deposits: REPLY (2011)
    GSA, Geological Society of America
    In:  Geology, 39 (3). e236-e237.
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    Publication Date: 2017-05-18
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  • 8
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    Using B isotopes and B/Ca in corals from low saturation springs to constrain calcification mechanisms (2019)
    Nature Research
    In:  Nature Communications, 10 . Art.Nr. 3580.
    Details
    Publication Date: 2022-01-31
    Description: Ocean acidification is expected to negatively impact calcifying organisms, yet we lack understanding of their acclimation potential in the natural environment. Here we measured geochemical proxies (δ11B and B/Ca) in Porites astreoides corals that have been growing for their entire life under low aragonite saturation (Ωsw: 0.77–1.85). This allowed us to assess the ability of these corals to manipulate the chemical conditions at the site of calcification (Ωcf), and hence their potential to acclimate to changing Ωsw. We show that lifelong exposure to low Ωsw did not enable the corals to acclimate and reach similar Ωcf as corals grown under ambient conditions. The lower Ωcf at the site of calcification can explain a large proportion of the decreasing P. astreoides calcification rates at low Ωsw. The naturally elevated seawater dissolved inorganic carbon concentration at this study site shed light on how different carbonate chemistry parameters affect calcification conditions in corals.
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  • 9
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    Reorganization of the North Atlantic Oscillation during early Holocene deglaciation (2016)
    Nature Research
    Details
    Publication Date: 2023-08-01
    Description: The North Atlantic Oscillation is the dominant atmospheric pressure mode in the North Atlantic region and affects winter temperature and precipitation in the Mediterranean, northwest Europe, Greenland, and Asia1. The index that describes the sea-level pressure difference between Iceland and the Azores is correlated with a dipole precipitation pattern over northwest Europe and northwest Africa. How the North Atlantic Oscillation will develop as the Greenland ice sheet melts is unclear. A potential past analogue is the early Holocene, during which melting ice sheets around the North Atlantic, freshened surface waters, affecting the strength of the meridional overturning circulation. Here we present a Holocene rainfall record from northwest Africa based on speleothem δ18O and compare it against a speleothem-based rainfall record from Europe. The two records are positively correlated during the early Holocene, followed by a shift to an anti-correlation, similar to the modern record, during the mid-Holocene. On the basis of our simulations with an Earth system model, we suggest the shift to the anti-correlation reflects a large-scale atmospheric and oceanic reorganization in response to the demise of the Laurentide ice sheet and a strong reduction of meltwater flux to the North Atlantic, pointing to a potential sensitivity of the North Atlantic Oscillation to the melting of ice sheets.
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  • 10
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    Southern Ocean contribution to both steps in deglacial atmospheric CO2 rise (2021)
    Nature Research
    Details
    Publication Date: 2024-02-07
    Description: The transfer of vast amounts of carbon from a deep oceanic reservoir to the atmosphere is considered to be a dominant driver of the deglacial rise in atmospheric CO2. Paleoceanographic reconstructions reveal evidence for the existence of CO2-rich waters in the mid to deep Southern Ocean. These water masses ventilate to the atmosphere south of the Polar Front, releasing CO2 prior to the formation and subduction of intermediate-waters. Changes in the amount of CO2 in the sea water directly affect the oceanic carbon chemistry system. Here we present B/Ca ratios, a proxy for delta carbonate ion concentrations Δ[CO32−], and stable isotopes (δ13C) from benthic foraminifera from a sediment core bathed in Antarctic Intermediate Water (AAIW), offshore New Zealand in the Southwest Pacific. We find two transient intervals of rising [CO32−] and δ13C that that are consistent with the release of CO2 via the Southern Ocean. These intervals coincide with the two pulses in rising atmospheric CO2 at ~ 17.5–14.3 ka and 12.9–11.1 ka. Our results lend support for the release of sequestered CO2 from the deep ocean to surface and atmospheric reservoirs during the last deglaciation, although further work is required to pin down the detailed carbon transfer pathways.
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