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Quantitative records of bottom water oxygen (BWO) are critical for understanding deep ocean change through time. Because of the stoichiometric relationship between oxygen and carbon, BWO records provide insight into the physical and biogeochemical processes that control the air‐sea partitioning of both gases with important implications for climate over Quaternary glacial‐interglacial cycles. Here, we present new geochemical data sets from Ocean Discovery Program Site 1240 in the eastern equatorial Pacific to constrain paleoproductivity (Ba_xsflux) and BWO using a multiproxy approach (aU, Mn/Al, Δδ¹³C, and U/Ba). This combination of approaches allows us to quantitatively identify changes in BWO and to parse local and basin‐wide contributions to the signal. We find that upwelling, not dust input, is responsible for driving productivity changes at the site. Changes in local carbon export are not the primary driver of changes in BWO, which instead reflect basin‐wide changes driven by processes in the Southern Ocean. Our BWO results provide direct evidence for the role of orbital precession and obliquity in driving deep sea respired carbon and oxygen concentrations. We find variations in BWO on the order of ∼50 μmol/kg that occur with ∼23 kyr periodicity during the substages of Marine Isotope Stage 5, and variations of ∼100 μmol/kg on glacial‐interglacial timescales. These findings have important implications for the role of insolation in driving deep ocean respired oxygen and carbon concentrations, and point to physical and biogeochemical changes in the Southern Ocean as key drivers of planetary‐scale carbon change. 

Abstract An important role in the cycling of marine trace elements is scavenging, their adsorption and removal from the water column by sinking particles. Boundary scavenging occurs when areas of strong particle flux drive preferential removal of the trace metals at locations of enhanced scavenging. Due to its uniform production and quick burial via scavenging,²³⁰Th is used to assess sedimentary mass fluxes; however, these calculations are potentially biased near regions where net lateral transport of dissolved²³⁰Th violates the assumption that the flux of particulate²³⁰Th to the seabed equals its rate of production in the water column. Here, we present a water column transect of dissolved²³⁰Th along 152° W between Alaska and Tahiti (GEOTRACES GP15), where we examine²³⁰Th profiles across multiple biogeochemical provinces and, novelly, the lateral transport of²³⁰Th to distal East Pacific Rise hydrothermal plumes. We observed a strong relationship between the slope of dissolved²³⁰Th concentration‐depth profiles and suspended particle matter inventory in the upper‐mid water column, reinforcing the view that biogenic particle mass flux sets the background²³⁰Th distribution in open ocean settings. We find that, instead of the region of enhanced particle flux around the equator, hydrothermal plumes act as a regional boundary sink of²³⁰Th. At 152° W, we found that the flux‐to‐production ratio, and thereby error in²³⁰Th‐normalized sediment flux, is between 0.80 and 1.50 for hydrothermal water, but the error is likely larger approaching the East Pacific Rise. 

Abstract The quantity and preservation of carbon‐rich organic matter (OM) underlying permafrost uplands, and the evolution of carbon accumulation with millennial climate change, are large sources of uncertainty in carbon cycle feedbacks on climate change. We investigated permafrost OM accumulation and degradation over the Holocene using a transect of sediment cores dating back to at least c. 6 ka, from a hillslope in the Eight Mile Lake watershed, central Alaska. We find decimeter‐scale organic‐rich (111 ± 45 kg C m⁻³) and organic‐poor (49 ± 30 kg C m⁻³) layers below an upper peat, which store 35% ± 11% and 41% ± 20% of the carbon in the upper 1 m, respectively. In organic‐poor layers, scattered¹⁴C ages of plant macrofossils and higher percentages of degradedAlnusandBetulapollen indicate reworking by cryoturbation and hillslope processes. Whereas organic carbon to nitrogen ratios generally indicate OM freshening up‐core, amino acid bacterial biomarkers, includingd‐enantiomers and gamma‐aminobutyric acid, suggest enhanced degradation prior to 5 ka. Carbon accumulation rates increased from ∼4 to 14 g C m⁻² year⁻¹from c. 8 to 0.2 ka, coinciding with decreasing temperatures and increasing moisture regionally, which may have promoted OM accumulation. Carbon stocks within the upper 1 m average 66 ± 13 kg C m⁻², varying from 77 kg C m⁻²in a buried depression on the upper slope to 48 kg C m⁻²downslope. We conclude that heterogeneity in preserved OM reflects a combination of hillslope geomorphic processes, cryoturbation, and climatic variations over the Holocene. 

Abstract The tip of the red giant branch (TRGB) provides a luminous standard candle for constructing distance ladders to measure the Hubble constant. In practice, its measurements via edge-detection response (EDR) are complicated by the apparent fuzziness of the tip and the multipeak landscape of the EDR. Previously, we optimized an unsupervised algorithm, Comparative Analysis of TRGBs, to minimize the variance among multiple halo fields per host without relying on individualized choices, achieving state-of-the-art ∼<0.05 mag distance measures for optimal data. Here we apply this algorithm to an expanded sample of SN Ia hosts to standardize these to multiple fields in the geometric anchor, NGC 4258. In concert with the Pantheon+ SN Ia sample, this analysis produces a (baseline) result ofH₀= 73.22 ± 2.06 km s⁻¹Mpc⁻¹. The largest difference inH₀between this and similar studies employing the TRGB derives from corrections for SN survey differences and local flows used in the most recent SN Ia compilations that were absent in earlier studies. The SN-related differences total ∼2.0 km s⁻¹Mpc⁻¹. A smaller share, ∼1.4 km s⁻¹Mpc⁻¹, results from the inhomogeneity of the TRGB calibration across the distance ladder. We employ a grid of 108 variants around the optimal TRGB algorithm and find that the median of the variants is 72.94 ± 1.98 km s⁻¹Mpc⁻¹with an additional uncertainty due to algorithm choices of 0.83 km s⁻¹Mpc⁻¹. None of these TRGB variants result in anH₀of less than 71.6 km s⁻¹Mpc⁻¹. 

The quantity and preservation of carbon-rich organic matter (OM) underlying permafrost uplands, and the evolution of carbon accumulation with millennial climate change, are large sources of uncertainty in carbon cycle feedbacks on climate change. We investigated permafrost OM accumulation and degradation over the Holocene using a transect of sediment cores dating back to at least c. 6-8 ka, from a hillslope in the Eight Mile Lake watershed, central Alaska. This dataset collected from four permafrost sediment cores includes a variety of biogeochemical datasets including radiocarbon, carbon, nitrogen, particle size, amino acids (concentrations and D:L), bulk density and water content. 

Abstract. Land cover governs the biogeophysical and biogeochemical feedbacks between the land surface and atmosphere. Holocene vegetation-atmosphere interactions are of particular interest, both to understand the climate effects of intensifying human land use and as a possible explanation for the Holocene Conundrum, a widely studied mismatch between simulated and reconstructed temperatures. Progress has been limited by a lack of data-constrained, quantified, and consistently produced reconstructions of Holocene land cover change. As a contribution to the Past Global Changes (PAGES) LandCover6k Working Group, we present a new suite of land cover reconstructions with uncertainty for North America, based on a network of 1445 sedimentary pollen records and the REVEALS pollen-vegetation model coupled with a Bayesian spatial model. These spatially comprehensive land cover maps are then used to determine the pattern and magnitude of North American land cover changes at continental to regional scales. Early Holocene afforestation in North America was driven by rising temperatures and deglaciation, and this afforestation likely amplified early Holocene warming via the albedo effect. A continental-scale mid-Holocene peak in summergreen trees and shrubs (8.5 to 4 ka) is hypothesized to represent a positive and understudied feedback loop among insolation, temperature, and phenology seasonality. A last-millennium decrease in summergreen trees and shrubs with corresponding increases in open land likely was driven by a spatially varying combination of intensifying land use and neoglacial cooling. Land cover trends vary within and across regions, due to individualistic taxon-level responses to environmental change. Major species-level events, such as the mid-Holocene decline of eastern hemlock, may have altered regional climates. The substantial land-cover changes reconstructed here support the importance of biogeophysical vegetation feedbacks to Holocene climate dynamics. However, recent model experiments that invoke vegetation feedbacks to explain the Holocene Conundrum may have overestimated the land cover forcing by replacing Northern Hemisphere grasslands >30° N with forests; an ecosystem state that is not supported by these land cover reconstructions. These Holocene reconstructions for North America, along with similar LandCover6k products now available for other continents, serve the Earth system modeling community by providing better-constrained land cover scenarios and benchmarks for model evaluation, ultimately making it possible to better understand the regional- to global-scale processes driving Holocene land cover dynamics. 

Organic-rich surficial materials of the Arctic are a storehouse of frozen carbon (C) of global consequence. Climate ultimately controls the exchange of carbon between this reservoir and the atmosphere, but the long-term relation between climate and permafrost carbon is highly uncertain. This study draws from climate changes that occurred during the recent geologic past (late glacial and Holocene), which serve as natural experiments, to quantify the long-term (millennial-scale) relation between climate and the mass of carbon that accumulated under distinctly different climate states. Whereas previous studies of the effects of climate changes on permafrost carbon have generally focused on lakes and peatlands of low-lying terrain, this study provides complementary information from upland deposits that mantle hilly terrain, possibly the least-well known component of the arctic frozen organic carbon inventory. The project applies and develops new approaches to investigating the relation between climate and carbon sequestration in an understudied setting and at long time scales by bringing together expertise in Arctic paleoecology, paleoclimatology, surficial geology, geochronology and quantitative geomorphology. This paleo perspective provides a unique approach to help infer how permafrost and its C reservoir may react in the future.