Description: Limited constraints on the variability of the deep-water production in the Labrador Sea complicate reconstructions of the strength of the Atlantic Meridional Overturning Circulation (AMOC) during the Late Quaternary. Large volumes of detrital carbonates were repeatedly deposited in the Labrador Sea during the last 32 kyr, potentially affecting radiogenic Nd isotope signatures. To investigate this the Nd isotope compositions of deep and intermediate waters were extracted from the authigenic Fe-Mn oxyhydroxide fraction, foraminiferal coatings, the residual silicates and leachates of dolostone grains. We provide a first order estimation of Nd release via dissolution of detrital carbonates and its contribution to the authigenic ԑNd signatures in the Labrador Sea. During the Last Glacial Maximum the Nd isotope signatures in the Labrador Sea would allow active water mass mixing with more radiogenic ɛNd values (-12.6 and -14) prevailing in its eastern part whereas less radiogenic values (ɛNd ∼ -18.4) were found on the western Labrador slope. The deposition of detrital carbonates during Heinrich stadials (2,1) was accompanied by negative detrital and authigenic Nd isotope excursions (ɛNd ∼ -31) that were likely controlled by dissolution of dolostone or dolostone associated mineral inclusions. This highly unradiogenic signal dominated the authigenic phases and individual water masses in the Labrador Sea, serving as potential source of highly unradiogenic Nd to the North Atlantic region, while exported southward. The Holocene authigenic ɛNd signatures of the coatings and leachates significantly differed from those of the detrital silicates, approaching modern bottom water mass signatures during the Late Holocene. Key Points - Estimation of Nd release via dissolution of detrital carbonates and its contribution to the authigenic ԑNd signatures in the Labrador Sea - Dissolution of detrital dolostones in the water column during Heinrich stadials at least partially controlled ɛNd signatures - During the LGM generally more radiogenic signatures possibly indicate active water mass advection and mixing in the Labrador Sea

Description: Highlights • Porewater calcite dissolution may have occurred during the deglacial interval. • There is a significant decoupling of coccolith and alkenone concentrations in core 004. • Non-calcifying haptophytes most likely produced the alkenones in the glacial interval. Abstract The important changes that took place in the glacial cycle at the termination, from the Last Glacial Maximum to the present interglacial, deserve an examination of ocean sedimentary records that document past productivity, carbon fluxes, and carbonate preservation. In this study, we analyzed coccoliths, alkenones, and foraminifers in core HU2008–029-004 PC (61.46°N and 58.04°W, water depth = 2,674 m) from the northwestern Labrador Sea to document linkages between hydrographic conditions, biogenic carbonate fluxes to the seafloor, and their preservation/dissolution during the last 25,000 years. Large changes in coccolith and foraminifer concentrations are recorded, with sediments from the last glacial interval containing significantly less carbonate microfossils (9.5 ± 3.9 × 105 coccoliths g−1 and 2,860 ± 580 planktonic foraminifers g−1) than sediments from the deglacial and postglacial intervals (up to 3.1 × 108 coccoliths g−1 and 2.9 × 104 foraminifers g−1). Three foraminifer-based calcite dissolution indices were used to evaluate biogenic carbonate preservation: the planktonic foraminifer fragmentation index, the ratio of benthic-to-planktonic foraminifers (B/P), and the ratio of organic linings to benthic foraminifers (OL/B). Fragmentation remained low throughout the postglacial (mean of 4%) but reached up to 8% in the deglacial and peaked at 16% in samples from the Bølling-Allerød of the late glacial interval. Samples from the Bølling-Allerød and the deglacial interval also display a slightly elevated B/P index (〉0.15), which suggests that some dissolution may have occurred. In contrast, with the exception of the Bølling-Allerød and the deglacial interval, near zero OL/B values characterize most of the sequence, suggesting good biogenic carbonate preservation, which implies that the low biogenic carbonate and coccolith content in sediments of the glacial stage mirror low productivity of calcifying organisms. The elevated fragmentation of foraminifers during the Bølling-Allerød and the deglacial interval, a time of elevated productivity and low percentages of ice-rafted debris, may indicate the development of calcite undersaturated porewaters and consequent dissolution resulting from oxic remineralization of sedimentary organic matter. We also identify a significant decoupling of coccolith and alkenone concentrations throughout the core. Colder-than-expected UK37-SST estimates from the alkenones of the glacial interval rule out possible allochthonous inputs from lower-latitude locations. Instead, our records imply that at least during the glacial interval, alkenones were produced by non-calcifying haptophytes that may not follow the canonical UK37-based temperature calibrations.

Description: Subglacially precipitated carbonate crusts (SPCCs) formed on bedrock and till boulder surfaces adjacent to the Barnes Ice Cap (BIC), central Baffin Island, Arctic Canada, act as unique archives of Laurentide Ice Sheet basal conditions. Uranium-series dating of these features reveals that carbonate precipitation from subglacial meltwater occurred during the Last Glacial Maximum (LGM), requiring warm-based ice in the region at that time. However, the preservation of fragile SPCCs is unlikely beneath erosive warm-based ice, suggesting that the transition to subsequent cold-based conditions took place shortly after the LGM, and glacial erosion in the region occurred dominantly prior to the LGM. The oxygen isotopic composition of the meltwater from which the SPCCs precipitated is indistinguishable from that of the debris-rich BIC basal ice (d18O –24‰ referenced to Vienna standard mean ocean water), but distinct from that of the overlying white Pleistocene ice (d18O ~–35‰), demonstrating that SPCCs are reliable archives of the isotopic composition of only the basal ice of past ice sheets.

Description: Increasing warming causes severe environmental changes in the Arctic coastal zone such as longer open water periods and rising sea levels. These processes intensify the erosion of permafrost coasts and lateral transport of sediment and organic matter (OM). Lagoons play a particularly important role in the transfer process of terrestrial OM but have been rarely investigated in the Arctic. Here, we studied a lagoon system along the Arctic Yukon coast to better understand the lateral pathways of OM from land to sea and its deposition dynamics over time. We sampled terrestrial, lagoon and marine sediment to track OM along a land-lagoon-ocean transect and took short cores to assess OM deposition dynamics. Samples were analysed for total organic carbon and nitrogen (TOC, TN), stable carbon and nitrogen isotopes (δ13C, δ15N), as well as grain size and surface area. We further analysed the shoreline change rates of the lagoon from 1950s to 2018 and coupled it to sedimentation rates derived from 210Pb/137Cs dating. Turbidity was estimated in the lagoon surface water using Landsat imagery for the main wind directions. Our results show that OC concentrations significantly decrease along the land-lagoon-ocean transect. Currents potentially removed large portions of eroded OM, especially under easterly winds, which is indicated by elevated SPM concentrations. In contrast, OM can get buried quickly, which is indicated by high OM contents in deeper lagoon sediments. Coastal erosion rates in the lagoon increased drastically since the 1970s and correspond with increasing sedimentation rates, suggesting a direct relation of environmental forcing and OM deposition dynamics in the lagoon. We conclude that lagoons are a crucial transfer zone between land and ocean, which can substantially influence OM pathways. Under current environmental change scenarios in the Arctic, the role of lagoons may get more important as gateways of OM from land to sea.

Description: 230Th‐normalization is a valuable paleoceanographic tool for reconstructing high‐resolution sediment fluxes during the late Pleistocene (last ~500,000 years). As its application has expanded to ever more diverse marine environments, the nuances of 230Th systematics, with regards to particle type, particle size, lateral advective/diffusive redistribution, and other processes, have emerged. We synthesized over 1000 sedimentary records of 230Th from across the global ocean at two time slices, the Late Holocene (0‐5000 years ago, or 0‐5 ka) and the Last Glacial Maximum (18.5‐23.5 ka), and investigated the spatial structure of 230Th‐normalized mass fluxes. On a global scale, sedimentary mass fluxes were significantly higher during the Last Glacial Maximum (1.79‐2.17 g/cm2kyr, 95% confidence) relative to the Holocene (1.48‐1.68 g/cm2kyr, 95% confidence). We then examined the potential confounding influences of boundary scavenging, nepheloid layers, hydrothermal scavenging, size dependent sediment fractionation, and carbonate dissolution on the efficacy of 230Th as a constant flux proxy. Anomalous 230Th behavior is sometimes observed proximal to hydrothermal ridges and in continental margins where high particle fluxes and steep continental slopes can lead to the combined effects of boundary scavenging and nepheloid interference. Notwithstanding these limitations, we found that 230Th‐normalization is a robust tool for determining sediment mass accumulation rates in the majority of pelagic marine settings (〉 1000 m water depth).

Description: AtlanticWater (AW) advection plays an important role in climatic, oceanographic and environmental conditions in the eastern Arctic. Situated along the only deep connection between the Atlantic and the Arctic oceans, the Svalbard Archipelago is an ideal location to reconstruct the past AW advection history and document its linkage with local glacier dynamics, as illustrated in the present study of a 275 cm long sedimentary record from Woodfjorden (northern Spitsbergen; water depth: 171 m) spanning the last �15 500 years. Sedimentological, micropalaeontological and geochemical analyses were used to reconstruct changes in marine environmental conditions, sea ice cover and glacier activity. Data illustrate a partial break-up of the Svalbard–Barents Sea Ice Sheet from Heinrich Stadial 1 onwards (until �14.6 ka). During the Bølling–Allerød (�14.6–12.7 ka), AW penetrated as a bottom water mass into the fjord system and contributed significantly to the destabilization of local glaciers. During the Younger Dryas (�12.7–11.7 ka), it intruded into intermediate waters while evidence for a glacier advance is lacking. A short-term deepening of the halocline occurred at the very end of this interval. During the early Holocene (�11.7–7.8 ka), mild conditions led to glacier retreat, a reduced sea ice cover and increasing sea surface temperatures, with a brief interruption during the Preboreal Oscillation (�11.1–10.8 ka). Due to a �6000-year gap, the mid- Holocene is not recorded in this sediment core. During the late Holocene (�1.8–0.4 ka), a slightly reduced AW inflow and lower sea surface temperatures compared to the early Holocene are reconstructed. Glaciers, which previously retreated to the shallower inner parts of the Woodfjorden system, likely advanced during the late Holocene. In particular, topographic control in concert with the reduced summer insolation partly decoupled glacier dynamics from AW advection during this recent interval.