Correcting a systematic bias in an ocean drilling project site 882 alkenone sea surface temperature record

A global atmospheric methane record from a tropical ice core.

Tropical wetlands are widely considered the largest natural source of atmospheric methane (CH4)1-4. However, uncertainties about wetland extent and CH4 production have led to large variations in modelled CH4 emission trends2,3. Most historical reconstructions rely on data from polar ice cores, which cannot fully resolve the tropical contribution5,6 to the CH4 budget. Here we present a 2,000-year record of atmospheric CH4 concentrations from ice cores drilled from the South Peak summit of Nevado Huascarán (Summit Core A, SCA; -9.122° S, -77.605° W; 6,768 m asl). We find that the trends and magnitudes of our CH4 record are broadly consistent with polar records7. Our δ13C-CH4 measurements (from approximately 1530 CE to 1999 CE) align with isotope values8 consistent with a dominant tropical CH4 source. Integration of our record into an atmospheric four-box model suggests a sustained equatorial dominance of CH4 source strength over the past two millennia. Our findings indicate that equatorial CH4 emissions are higher than previous estimates based only on polar ice core data, supporting the long-standing hypothesis that low-latitude CH4 emissions dominated pre-industrial (PI) CH4 variability5,6. These results demonstrate the importance of tropical ice cores on the reconstruction of CH4 variability and latitudinal distribution.

Paleo Data Assimilation of Coral δ18O Part 1: Best Practices and Uncertainties

Uncertainties in early 20th‐century observations limit understanding of the full range of internal climate variability and muddle understanding of climate system response to external forcing. Coral archives can provide high‐resolution information about historical sea‐surface temperatures (SSTs) and hydroclimatic change over the tropical oceans. Paleoclimate data assimilation (paleo‐DA) can extract information from these valuable records and generate gridded, full‐field reconstructions. However, reasonable choices in the paleo‐DA methodology can influence the reconstructed climate variability. Here, we outline the challenges and opportunities for using the oxygen isotope composition of corals (δ18O) to reconstruct temperature and hydrological cycle variability with paleo‐DA using the Last Millennium Reanalysis offline framework. Using a proxy network of coral δ18O and Sr/Ca that spans the 20th century, we evaluate the sensitivity of reconstructions to the choice of coral δ18O proxy system models (PSMs), the choice of instrumental sea surface salinity (SSS) products with the PSMs, and the choice of a climate model prior, ultimately testing 28 distinct reconstruction approaches. The choice of coral δ18O PSM and climate model prior can generate inconsistencies in the reconstructions. This influence is likely due to sparse observations of δ18Osw and SSS limiting “training” data, the nonstationary relationship between SSS and δ18Osw, and the strong covariability of SST and SSS across the global tropics. Given these limitations, we advise the use of a high‐resolution climate model prior, but the “best” configuration of PSM, SSS product, and prior is dependent on the question and region of interest.

Deep-water hydrographic variability in the Northeast Atlantic during MIS 4

Holocene sea-surface temperature as a regional driver of environmental shifts and its effects on humans of La Paz Bay, Gulf of California

Past oceanic characteristics can be reconstructed via δ 18 O in bivalve shells, enabling the study of climatic patterns that affected marine and human communities. Through the novel analysis of δ 18 O in Pinctada mazatlanica shells from a 14 C dated Holocene coastal camp we calculated Holocene summer SSTs of La Paz Bay, Baja California Sur, México. Presently summer δ 18 O of P. mazatlanica is −1.8 ± 0.2‰, varying between −1.3‰ and −1.9‰ during the last 8.4 ka. For the years 8413 BP, 7726 BP and 7046 BP, δ 18 O was 0.2–0.1‰ higher than current values, suggesting slightly colder sea surface temperature (SST), while in 7795 BP, values were 0.1‰ lower, suggesting slightly warmer SST. In 6936 BP and 2093 BP, values were 0.4–0.5‰ higher, suggesting significantly colder SSTs. Periods of low (high) SST were characterized by arid (humid) conditions and high (low) marine primary productivity (MPP), favouring human presence (absence) throughout the Holocene. Our results, like those of other paleoceanographic studies, align with orbital forcing and its effects on SST and MPP, which affected human settlement patterns at regional and global scales.

Climate warming and atmospheric deposition jointly accelerate the Antarctic Peninsula atmosphere-glacier-land-ocean mercury loop.

Mercury (Hg) is a toxic pollutant of global concern that threatens ecosystem and human health. Its cycle is being jointly reshaped by anthropogenic emissions and polar warming, yet the response of the Antarctic system remains poorly constrained. Here, we reconstruct the past 200 y of Hg source-sink dynamics in the Antarctic Peninsula (AP), Antarctica's fastest-warming region with the most pronounced glacier melting, by combining geochemical and isotopic records from 16 sediment cores collected across the AP shelf, with a developed observation-constrained multimedia Hg budget model. We find that despite its remoteness from anthropogenic emission sources, the modern AP shelf exhibits an Hg accumulation rate of 93 ± 58 µg m-2 y-1, twice the global shelf average. Since industrialization, this accumulation rate has increased by 160%, making the AP one of the major hotspots of marine Hg enrichment. The model further reveals that this acceleration is driven by two coupled mechanisms: 1) enhanced atmospheric deposition and expanding open water strengthen the direct uptake of atmospheric Hg by seawater within the atmosphere-ocean loop, and 2) ice melt and erosion activate the long-overlooked atmosphere-glacier-land-ocean loop, remobilizing legacy Hg stored on land. The coupling of these two loops has increased ice melt-driven terrestrial Hg release by 550% and air-sea exchange by 350%. Thus, climate warming is turning the large historical Hg reservoir in Antarctica into an active secondary pollution source, amplifying polar Hg pollution risk.

Middle to Late Miocene paleoceanographic evolution of the south-eastern Indian Ocean 1 (