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Organic matter represents a large pool of carbon in the ocean. Radiocarbon and chemical analyses suggest that larger particles are preferentially remineralized in the Pacific Ocean, with smaller particles and molecules persisting longer. Marine organic matter is one of Earth’s largest actively cycling reservoirs of organic carbon and nitrogen 1 , 2 . The processes controlling organic matter production and removal are important for carbon and nitrogen biogeochemical cycles, which regulate climate. However, the many possible cycling mechanisms have hindered our ability to quantify marine organic matter transformation, degradation and turnover rates 3 , 4 . Here we analyse existing and new measurements of the carbon:nitrogen ratio and radiocarbon age of organic matter spanning sizes from large particulate organic matter to small dissolved organic molecules. We find that organic matter size is negatively correlated with radiocarbon age and carbon:nitrogen ratios in coastal, surface and deep waters of the Pacific Ocean. Our measurements suggest that organic matter is increasingly chemically degraded as it decreases in size, and that small particles and molecules persist in the ocean longer than their larger counterparts. Based on these correlations, we estimate the production rates of small, biologically recalcitrant dissolved organic matter molecules at 0.11–0.14 Gt of carbon and about 0.005 Gt of nitrogen per year in the deep ocean. Our results suggest that the preferential remineralization of large over small particles and molecules is a key process governing organic matter cycling and deep ocean carbon storage.

Climate change is predicted to alter marine phytoplankton communities and affect productivity, biogeochemistry, and the efficacy of the biological pump. We reconstructed high-resolution records of changing plankton community composition in the North Pacific Ocean over the past millennium. Amino acid–specific δ¹³C records preserved in long-lived deep-sea corals revealed three major plankton regimes corresponding to Northern Hemisphere climate periods. Non–dinitrogen-fixing cyanobacteria dominated during the Medieval Climate Anomaly (950–1250 Common Era) before giving way to a new regime in which eukaryotic microalgae contributed nearly half of all export production during the Little Ice Age (~1400–1850 Common Era). The third regime, unprecedented in the past millennium, began in the industrial era and is characterized by increasing production by dinitrogen-fixing cyanobacteria. This picoplankton community shift may provide a negative feedback to rising atmospheric carbon dioxide concentrations.

Photosynthesis in the surface ocean converts atmospheric CO2 into organic particles, with the fraction sinking to depth representing a major part of the ocean's biological pump. Although sinking particles are known to be altered by attached‐bacteria during transit, most prior organic geochemical data indicated only minor replacement of plankton‐derived particles by bacterial material. We exploit bacteria‐specific biomarkers (d‐amino acids) in a multi‐year sediment trap in the Pacific Ocean (1,200 m) and suggest a different view. Major d‐amino acids were consistently measured at abundance demonstrating widespread accumulation of bacterial material in sinking particles. Bacterial detritus was estimated to account for up to 19% of particulate organic carbon and up to 36% of particulate nitrogen, much higher than cell count‐based values. The bacterial relative contribution increased with decreasing export production. Our results indicate that bacterial material constitutes an underappreciated component of the biological pump, a role expected to rise as the ocean warms. Plain Language Summary Phytoplankton photosynthesis in the surface ocean plays a critical role in stabilizing atmospheric CO2. It converts CO2 into organic particles that sink and are reworked by colonizing bacteria. Bacteria respire most particles back to CO2 while also transforming some into their cell components. Although the involvement of bacteria can replace the plankton‐derived particles to bacterial material, most past organic geochemical data have suggested that the deep‐sea particles are still comprised mainly of plankton remnants. This renders the contribution of bacterial material to total particle export an unresolved and yet important question, because the source and composition of particles are important to their fate in the ocean. Here, we analyzed bacteria‐specific molecules in deep‐sea sinking particles and found that bacterial organic matter actually made up a large fraction of the particles. In addition, the relative contribution of bacterial material to the sinking particles increased as the total carbon export decreased. This has important implications for the future ocean carbon cycle, because modeling work predicts a scenario of lower carbon export to the deep sea as the ocean warms. In this context, our findings imply a greater importance of bacteria in marine organic matter export and sequestration in a warming ocean. Key Points We exploit d‐amino acid biomarkers in multi‐year deep‐sea sediment trap time series to evaluate bacterial contribution to total export Bacterial detritus accounts for up to 19 ± 8% of sinking POC and up to 36 ± 14% of PN, making up a large unrecognized part of biological pump The relative contribution of bacterial detritus to sinking particles increases with decreased export production

Stable isotope biochemistry (δ¹³C and δ¹⁵N) and radiocarbon dating of ancient human and animal bone document 2 distinct phases of plant and animal domestication at the Dadiwan site in northwest China. The first was brief and nonintensive: at various times between 7900 and 7200 calendar years before present (calBP) people harvested and stored enough broomcorn millet (Panicum miliaceum) to provision themselves and their hunting dogs (Canis sp.) throughout the year. The second, much more intensive phase was in place by 5900 calBP: during this time both broomcorn and foxtail (Setaria viridis spp. italica) millets were cultivated and made significant contributions to the diets of people, dogs, and pigs (Sus sp.). The systems represented in both phases developed elsewhere: the earlier, low-intensity domestic relationship emerged with hunter-gatherers in the arid north, while the more intensive, later one evolved further east and arrived at Dadiwan with the Yangshao Neolithic. The stable isotope methodology used here is probably the best means of detecting the symbiotic human-plant-animal linkages that develop during the very earliest phases of domestication and is thus applicable to the areas where these connections first emerged and are critical to explaining how and why agriculture began in East Asia.

Despite a reduction in nutrient supply to the North Pacific subtropical gyre, it has undergone a recent increase in nitrogen fixation, and here records of nitrogen isotopes preserved in Hawaiian corals show that this is a trend that could be linked to climate change since the end of the Little Ice Age. Corals record a North Pacific nitrogen boost The deep-sea Hawaiian gold coral Kulamanamana haumeaae is a remarkably long-lived species, sometimes attaining ages of thousands of years. Over the millennia these corals provide a unique geochemical time-series as they convert sinking phytoplankton and other tiny particles into a proteinaceous coral skeleton. Primary productivity in the North Pacific subtropical gyre has increased in recent decades despite a decline in nutrient supply. An ecosystem shift towards nitrogen-fixing plankton communities has been put forward as a possible explanation, but the cause for this shift remains unclear. Owen Sherwood and colleagues use nitrogen isotopic (δ 15 N) records from K. haumeaae corals to establish that the increase in nitrogen fixation had already began around 150 years ago, and that it may have been linked to Northern Hemisphere climate change since the end of the Little Ice Age. The North Pacific subtropical gyre (NPSG) plays a major part in the export of carbon and other nutrients to the deep ocean 1 . Primary production in the NPSG has increased in recent decades despite a reduction in nutrient supply to surface waters 2 , 3 . It is thought that this apparent paradox can be explained by a shift in plankton community structure from mostly eukaryotes to mostly nitrogen-fixing prokaryotes 2 , 3 , 4 . It remains uncertain, however, whether the plankton community domain shift can be linked to cyclical climate variability or a long-term global warming trend 5 . Here we analyse records of bulk and amino-acid-specific 15 N/ 14 N isotopic ratios (δ 15 N) preserved in the skeletons of long-lived deep-sea proteinaceous corals collected from the Hawaiian archipelago; these isotopic records serve as a proxy for the source of nitrogen-supported export production through time. We find that the recent increase in nitrogen fixation is the continuation of a much larger, centennial-scale trend. After a millennium of relatively minor fluctuation, δ 15 N decreases between 1850 and the present. The total shift in δ 15 N of −2 per mil over this period is comparable to the total change in global mean sedimentary δ 15 N across the Pleistocene–Holocene transition, but it is happening an order of magnitude faster 6 . We use a steady-state model and find that the isotopic mass balance between nitrate and nitrogen fixation implies a 17 to 27 per cent increase in nitrogen fixation over this time period. A comparison with independent records 7 , 8 suggests that the increase in nitrogen fixation might be linked to Northern Hemisphere climate change since the end of the Little Ice Age.

Deep-sea corals are found on hard substrates on seamounts and continental margins worldwide at depths of 300 to [almost equal to]3,000 m. Deep-sea coral communities are hotspots of deep ocean biomass and biodiversity, providing critical habitat for fish and invertebrates. Newly applied radiocarbon age dates from the deep water proteinaceous corals Gerardia sp. and Leiopathes sp. show that radial growth rates are as low as 4 to 35 μm year⁻¹ and that individual colony longevities are on the order of thousands of years. The longest-lived Gerardia sp. and Leiopathes sp. specimens were 2,742 years and 4,265 years, respectively. The management and conservation of deep-sea coral communities is challenged by their commercial harvest for the jewelry trade and damage caused by deep-water fishing practices. In light of their unusual longevity, a better understanding of deep-sea coral ecology and their interrelationships with associated benthic communities is needed to inform coherent international conservation strategies for these important deep-sea habitat-forming species.

Caribbean Acropora spp. corals have undergone a decline in cover since the second half of the twentieth century. Loss of these architecturally complex and fast-growing corals has resulted in significant, cascading changes to the character, diversity, and available eco-spaces of Caribbean reefs. Few thriving Acropora spp. populations exist today in the Caribbean and western North Atlantic seas, and our limited ability to access data from reefs assessed via long-term monitoring efforts means that reef scientists are challenged to determine resilience and longevity of existing Acropora spp. reefs. Here we used multiple dating methods to measure reef longevity and determine whether Coral Gardens Reef, Belize, is a refuge for Acropora cervicornis against the backdrop of wider Caribbean decline. We used a new genetic-aging technique to identify sample sites, and radiocarbon and high-precision uranium-thorium (U-Th) dating techniques to test whether one of the largest populations of extant A. cervicornis in the western Caribbean is newly established after the 1980s, or represents a longer-lived, stable population. We did so with respect for ethical sampling of a threatened species. Our data show corals ranging in age from 1910 (14C) or 1915 (230Th) to at least November 2019. While we cannot exclude the possibility of short gaps in the residence of A. cervicornis earlier in the record, the data show consistent and sustained living coral throughout the 1980s and up to at least 2019. We suggest that Coral Gardens has served as a refuge for A. cervicornis and that identifying other, similar sites may be critical to efforts to grow, preserve, conserve, and seed besieged Caribbean reefs.

Carbon dioxide release during deglaciation At the end of the last ice age, rising atmospheric carbon dioxide levels coincided with a decline in carbon-14 levels, suggesting the release of very 'old' (radiocarbon-depleted) carbon dioxide from the deep ocean to the atmosphere. Rose et al . present radiocarbon records of surface and intermediate depth waters from two sediment cores in the southwest Pacific and Southern Ocean, and find a steady 170 per mil decrease in Δ 14 C that precedes and roughly equals in magnitude the decrease in the atmospheric radiocarbon signal during the early stages of the glacial–interglacial climatic transition. The initial rise in carbon dioxide levels may have originated from intermediate Southern Ocean water masses that were not strongly depleted in radiocarbon, followed by the release of radiocarbon-depleted carbon dioxide from deeper North Pacific waters. At the end of the last ice age, rising atmospheric CO 2 levels coincided with a decline in radiocarbon activity, suggesting the release of highly radiocarbon-depleted CO 2 from the deep ocean to the atmosphere. These authors present radiocarbon records of surface and intermediate-depth waters from two sediment cores and find an decrease in radiocarbon activity that precedes and roughly equals in magnitude the decrease in the atmospheric radiocarbon signal during the early stages of the glacial–interglacial climatic transition. Radiocarbon in the atmosphere is regulated largely by ocean circulation, which controls the sequestration of carbon dioxide (CO 2 ) in the deep sea through atmosphere–ocean carbon exchange. During the last glaciation, lower atmospheric CO 2 levels were accompanied by increased atmospheric radiocarbon concentrations that have been attributed to greater storage of CO 2 in a poorly ventilated abyssal ocean 1 , 2 . The end of the ice age was marked by a rapid increase in atmospheric CO 2 concentrations 2 that coincided with reduced 14 C/ 12 C ratios (Δ 14 C) in the atmosphere 3 , suggesting the release of very ‘old’ ( 14 C-depleted) CO 2 from the deep ocean to the atmosphere 3 . Here we present radiocarbon records of surface and intermediate-depth waters from two sediment cores in the southwest Pacific and Southern oceans. We find a steady 170 per mil decrease in Δ 14 C that precedes and roughly equals in magnitude the decrease in the atmospheric radiocarbon signal during the early stages of the glacial–interglacial climatic transition. The atmospheric decrease in the radiocarbon signal coincides with regionally intensified upwelling and marine biological productivity 4 , suggesting that CO 2 released by means of deep water upwelling in the Southern Ocean lost most of its original depleted- 14 C imprint as a result of exchange and isotopic equilibration with the atmosphere. Our data imply that the deglacial 14 C depletion previously identified in the eastern tropical North Pacific 5 must have involved contributions from sources other than the previously suggested carbon release by way of a deep Southern Ocean pathway 5 , and may reflect the expanded influence of the 14 C-depleted North Pacific carbon reservoir across this interval. Accordingly, shallow water masses advecting north across the South Pacific in the early deglaciation had little or no residual 14 C-depleted signals owing to degassing of CO 2 and biological uptake in the Southern Ocean.

The lemurs of Madagascar represent a prodigious adaptive radiation. At least 17 species ranging from 11 to 160 kg have become extinct during the past 2000 years. The effect of this loss on contemporary lemurs is unknown. The concept of competitive release favours the expansion of living species into vacant niches. Alternatively, factors that triggered the extinction of some species could have also reduced community-wide niche breadth. Here, we use radiocarbon and stable isotope data to examine temporal shifts in the niches of extant lemur species following the extinction of eight large-bodied species. We focus on southwestern Madagascar and report profound isotopic shifts, both from the time when now-extinct lemurs abounded and from the time immediately following their decline to the present. Unexpectedly, the past environments exploited by lemurs were drier than the protected (albeit often degraded) riparian habitats assumed to be ideal for lemurs today. Neither competitive release nor niche contraction can explain these observed trends. We develop an alternative hypothesis: ecological retreat, which suggests that factors surrounding extinction may force surviving species into marginal or previously unfilled niches.

High precision measurements of Δ14C were conducted for monthly samples of CO2from seven global stations over 2‐ to 16‐year periods ending in 2007. Mean Δ14C over 2005–07 in the Northern Hemisphere was 5 ‰ lower than Δ14C in the Southern Hemisphere, similar to recent observations from I. Levin. This is a significant shift from 1988–89 when Δ14C in the Northern Hemisphere was slightly higher than the South. The influence of fossil fuel CO2 emission and transport was simulated for each of the observation sites by the TM3 atmospheric transport model and compared to other models that participated in the Transcom 3 Experiment. The simulated interhemispheric gradient caused by fossil fuel CO2 emissions was nearly the same in both 1988–89 and 2005–07, due to compensating effects from rising emissions and decreasing sensitivity of Δ14C to fossil fuel CO2. The observed 5 ‰ shift must therefore have been caused by non‐fossil influences, most likely due to changes in the air‐sea14C flux in the Southern Ocean. Seasonal cycles with higher Δ14C in summer or fall were evident at most stations, with largest amplitudes observed at Point Barrow (71°N) and La Jolla (32°N). Fossil fuel emissions do not account for the seasonal cycles of Δ14C in either hemisphere, indicating strong contributions from non‐fossil influences, most likely from stratosphere‐troposphere exchange. Key Points Observations of 14CO2 were conducted at 7 sites Delta‐14C is now lower in the Northern Hemisphere, a shift from 1980s Gradients and seasonal cycles have substantial fossil and non‐fossil components