Multi-proxy evidence for sea level fall at the onset of the Eocene-Oligocene transition.

Continental-scale expansion of the East Antarctic Ice Sheet during the Eocene-Oligocene Transition (EOT) is one of the largest non-linear events in Earth's climate history. Declining atmospheric carbon dioxide concentrations and orbital variability triggered glacial expansion and strong feedbacks in the climate system. Prominent among these feedbacks was the repartitioning of biogeochemical cycles between the continental shelves and the deep ocean with falling sea level. Here we present multiple proxies from a shallow shelf location that identify a marked regression and an elevated flux of continental-derived organic matter at the earliest stage of the EOT, a time of deep ocean carbonate dissolution and the extinction of oligotrophic phytoplankton groups. We link these observations using an Earth System model, whereby this first regression delivers a pulse of organic carbon to the oceans that could drive the observed patterns of deep ocean dissolution and acts as a transient negative feedback to climate cooling.

Tracing the impacts of recent rapid sea ice changes and the A68 megaberg on the surface freshwater balance of the Weddell and Scotia Seas.

The Southern Ocean upper-layer freshwater balance exerts a global climatic influence by modulating density stratification and biological productivity, and hence the exchange of heat and carbon between the atmosphere and the ocean interior. It is thus important to understand and quantify the time-varying freshwater inputs, which is challenging from measurements of salinity alone. Here we use seawater oxygen isotopes from samples collected between 2016 and 2021 along a transect spanning the Scotia and northern Weddell Seas to separate the freshwater contributions from sea ice and meteoric sources. The unprecedented retreat of sea ice in 2016 is evidenced as a strong increase in sea ice melt across the northern Weddell Sea, with surface values increasing approximately two percentage points between 2016 and 2018 and column inventories increasing approximately 1 to 2 m. Surface meteoric water concentrations exceeded 4% in early 2021 close to South Georgia due to meltwater from the A68 megaberg; smaller icebergs may influence meteoric water at other times also. Both these inputs highlight the importance of a changing cryosphere for upper-ocean freshening; potential future sea ice retreats and increases in iceberg calving would enhance the impacts of these freshwater sources on the ocean and climate. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Finale: impact of the ORCHESTRA/ENCORE programmes on Southern Ocean heat and carbon understanding.

The 5-year Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports (ORCHESTRA) programme and its 1-year extension ENCORE (ENCORE is the National Capability ORCHESTRA Extension) was an approximately 11-million-pound programme involving seven UK research centres that finished in March 2022. The project sought to radically improve our ability to measure, understand and predict the exchange, storage and export of heat and carbon by the Southern Ocean. It achieved this through a series of milestone observational campaigns in combination with model development and analysis. Twelve cruises in the Weddell Sea and South Atlantic were undertaken, along with mooring, glider and profiler deployments and aircraft missions, all contributing to measurements of internal ocean and air-sea heat and carbon fluxes. Numerous forward and adjoint numerical experiments were developed and supported by the analysis of coupled climate models. The programme has resulted in over 100 peer-reviewed publications to date as well as significant impacts on climate assessments and policy and science coordination groups. Here, we summarize the research highlights of the programme and assess the progress achieved by ORCHESTRA/ENCORE and the questions it raises for the future. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Improving the routine analysis of siderite for δ13 C and δ18 O in environmental change research.

RATIONALE: The carbon (δ13 C) and oxygen (δ18 O) isotope composition of siderite (FeCO3 ) is used widely to understand and quantify geochemical processes to reconstruct past climate and environmental change. However, few laboratories follow precisely the same protocol for the preparation and analysis of siderite-bearing materials, which combined with the absence of international reference materials and mineral-specific acid fractionation factors, leads potentially to significant differences in isotope data generated by individual laboratories. Here we examine procedures for the isotope analysis of siderite and discuss factors potentially contributing to inconsistencies in sample measurement data. METHODS: Isotope analysis of siderite is first assessed using similar versions of the classical off-line, sealed vessel acid digestion method by comparing data sets obtained from intercomparison materials measured at two participating laboratories. We then compare data from the classical method against those generated using an automated preparation technique using data produced from an independent set of test materials. RESULTS: Measurement of siderite δ13 C is generally both repeatable and reproducible, but measurement of δ18 O may be subject to large (~1‰), method-dependent bias for siderite reacted at differing temperatures (70°C and 100°C) under classical and automated CO2 preparation conditions. The potential for poor oxygen isotope measurement reproducibility is amplified by local differences in sample preparation protocols and procedures used to calibrate measurement data to international reference scales. CONCLUSIONS: We offer suggestions for improving the repeatability and reproducibility of δ13 C and δ18 O analysis on siderite. The challenge of producing consistent isotope data from siderite can only be resolved by ensuring the availability of siderite reference materials to facilitate identical treatment as a basis for minimising method-dependent contributions to data inconsistency between laboratories.

In flux: Annual transport and deposition of suspended heavy metals and trace elements in the urbanised, tropical Red River Delta, Vietnam.

Due to the depositional environment, river deltas are said to act as filters and sinks for pollutants. However, many deltas are also densely populated and rapidly urbanizing, creating new and increased sources of pollutants. These sources pose the risk of tipping these environments from pollution sinks to sources, to the world's oceans. We provide detailed seasonal and annual assessments of metal contaminants in riverine suspended particulate matter (SPM) across the densely populated Red River Delta (RRD), Vietnam. The global contributions of elements from the RRD are all <0.2% with many elemental fluxes <0.01%, suggesting the RRD is not a major source of elemental pollution to the ocean. However, 'hotspots' of metal pollution due to human activity and the impacts of tropical storm Son Tinh (July 2018) exceed both national level regulations and international measures of toxicity (e.g. enrichment factors). There is widespread 'extreme pollution' of Cd (enrichment factor >40) and concentrations of As higher than national regulation limits (>17 mg/Kg) at all sites other than one upstream, agricultural-dominated tributary in the dry season. These 'hotspots' are characterised by high inputs of organic matter (e.g. manure fertiliser and urban wastewater), which influences elemental mobility in the particulate and dissolved phases, and are potentially significant sources of pollution downstream. In addition, in the marine and fresh water mixing zone, salinity effects metal complexation with organic matter increasing metals in the particulate phase. Our calculations indicate that the delta is currently acting as a pollutant sink (as determined by high levels of pollutant deposition ∼50%). However, increased in-washing of pollutants and future projected increases in monsoon intensity, saline intrusion, and human activity could shift the delta to become a source of toxic metals. We show the importance of monitoring environmental parameters (primarily dissolved organic matter and salinity) in the RRD to assess the risk of transport and accumulation of toxic metals in the delta sediments, which can lead to net-increases in anthropogenic pollution in the coastal zone and the incorporation of toxic elements in the food chain.

Extensive primary production promoted the recovery of the Ediacaran Shuram excursion.

Member IV of the Ediacaran Doushantuo Formation records the recovery from the most negative carbon isotope excursion in Earth history. However, the main biogeochemical controls that ultimately drove this recovery have yet to be elucidated. Here, we report new carbon and nitrogen isotope and concentration data from the Nanhua Basin (South China), where δ13C values of carbonates (δ13Ccarb) rise from - 7‰ to -1‰ and δ15N values decrease from +5.4‰ to +2.3‰. These trends are proposed to arise from a new equilibrium in the C and N cycles where primary production overcomes secondary production as the main source of organic matter in sediments. The enhanced primary production is supported by the coexisting Raman spectral data, which reveal a systematic difference in kerogen structure between depositional environments. Our new observations point to the variable dominance of distinct microbial communities in the late Ediacaran ecosystems, and suggest that blooms of oxygenic phototrophs modulated the recovery from the most negative δ13Ccarb excursion in Earth history.

Paleocene/Eocene carbon feedbacks triggered by volcanic activity.

The Paleocene-Eocene Thermal Maximum (PETM) was a period of geologically-rapid carbon release and global warming ~56 million years ago. Although modelling, outcrop and proxy records suggest volcanic carbon release occurred, it has not yet been possible to identify the PETM trigger, or if multiple reservoirs of carbon were involved. Here we report elevated levels of mercury relative to organic carbon-a proxy for volcanism-directly preceding and within the early PETM from two North Sea sedimentary cores, signifying pulsed volcanism from the North Atlantic Igneous Province likely provided the trigger and subsequently sustained elevated CO2. However, the PETM onset coincides with a mercury low, suggesting at least one other carbon reservoir released significant greenhouse gases in response to initial warming. Our results support the existence of 'tipping points' in the Earth system, which can trigger release of additional carbon reservoirs and drive Earth's climate into a hotter state.

Assessment of bias in carbon isotope composition of organic leaf matter due to pre-analysis milling methods.

RATIONALE: Stable isotope analysis of leaf material has many applications including assessment of plant water-use efficiency and palaeoclimatology. To facilitate interpretations of small shifts in the carbon isotope composition (δ13 C) of leaves, accurate and repeatable results are required. Pre-sample homogenisation is essential to ensure a representative sample is analysed, but can also introduce error. METHODS: We investigate how different grinding methods (freezer-milling and ball-milling) affect the carbon content and δ13 C of tree leaves from a wetland in Queensland, Australia, commenting on how increased temperature, sample contamination, sample loss or poor homogenisation may impact results. RESULTS: No alteration of leaf δ13 C is observed due to different milling methods, although there may be a significant increase in %C of samples processed using ball-milling. CONCLUSIONS: We suggest %C variability is possibly due to contamination from abraded plastic vials or insufficient homogenisation during ball-milling, with no significant impact on δ13 C. Overall, we suggest that intermittent ball-milling may be the best solution to reduce costs, preparation time and use of liquid nitrogen, aiming to achieve complete homogenisation using the shortest possible duration of milling.

Ventilation of the abyss in the Atlantic sector of the Southern Ocean.

The Atlantic sector of the Southern Ocean is the world's main production site of Antarctic Bottom Water, a water-mass that is ventilated at the ocean surface before sinking and entraining older water-masses-ultimately replenishing the abyssal global ocean. In recent decades, numerous attempts at estimating the rates of ventilation and overturning of Antarctic Bottom Water in this region have led to a strikingly broad range of results, with water transport-based calculations (8.4-9.7 Sv) yielding larger rates than tracer-based estimates (3.7-4.9 Sv). Here, we reconcile these conflicting views by integrating transport- and tracer-based estimates within a common analytical framework, in which bottom water formation processes are explicitly quantified. We show that the layer of Antarctic Bottom Water denser than 28.36 kg m[Formula: see text] [Formula: see text] is exported northward at a rate of 8.4 ± 0.7 Sv, composed of 4.5 ± 0.3 Sv of well-ventilated Dense Shelf Water, and 3.9 ± 0.5 Sv of old Circumpolar Deep Water entrained into cascading plumes. The majority, but not all, of the Dense Shelf Water (3.4 ± 0.6 Sv) is generated on the continental shelves of the Weddell Sea. Only 55% of AABW exported from the region is well ventilated and thus draws down heat and carbon into the deep ocean. Our findings unify traditionally contrasting views of Antarctic Bottom Water production in the Atlantic sector, and define a baseline, process-discerning target for its realistic representation in climate models.

A new automated method for high-throughput carbon and hydrogen isotope analysis of gaseous and dissolved methane at atmospheric concentrations.

RATIONALE: The dual isotope ratio analysis, carbon (δ13 C value) and hydrogen (δ2 H value), of methane (CH4 ) is a valuable tracer tool within a range of areas of scientific investigation, not least wetland ecology, microbiology, CH4 source identification and the tracing of geological leakages of thermogenic CH4 in groundwater. Traditional methods of collecting, purification, separating and analysing CH4 for δ13 C and δ2 H determination are, however, very time consuming, involving offline manual extractions. METHODS: Here we describe a new gas chromatography, pyrolysis/combustion, isotope ratio mass spectrometry (IRMS) system for the automated analysis of either dissolved or gaseous CH4 down to ambient atmospheric concentrations (2.0 ppm). Sample introduction is via a traditional XYZ autosampler, allowing either helium (He) purging of gas or sparging of water from a range of suitable, airtight bottles. RESULTS: The system routinely achieves precision of <0.3‰ for δ13 C values and <3.0‰ for δ2 H values, based on long-term replicate analysis of an in-house CH4 /He mix standard (BGS-1), corrected to two externally calibrated reference gases at near atmospheric concentrations of methane. Depending upon CH4 concentration and therefore bottle size, the system runs between 21 (140-mL bottle) and 200 samples (12-mL exetainer) in an unattended run overnight. CONCLUSIONS: This represents the first commercially available IRMS system for dual δ13 C and δ2 H analysis of methane at atmospheric concentrations and a step forward for the routine (and high-volume) analysis of CH4 in environmental studies.

Coupled impacts of sea ice variability and North Pacific atmospheric circulation on Holocene hydroclimate in Arctic Alaska.

Arctic Alaska lies at a climatological crossroads between the Arctic and North Pacific Oceans. The modern hydroclimate of the region is responding to rapidly diminishing sea ice, driven in part by changes in heat flux from the North Pacific. Paleoclimate reconstructions have improved our knowledge of Alaska's hydroclimate, but no studies have examined Holocene sea ice, moisture, and ocean-atmosphere circulation in Arctic Alaska, limiting our understanding of the relationship between these phenomena in the past. Here we present a sedimentary diatom assemblage and diatom isotope dataset from Schrader Pond, located ∼80 km from the Arctic Ocean, which we interpret alongside synthesized regional records of Holocene hydroclimate and sea ice reduction scenarios modeled by the Hadley Centre Coupled Model Version 3 (HadCM3). The paleodata synthesis and model simulations suggest the Early and Middle Holocene in Arctic Alaska were characterized by less sea ice, a greater contribution of isotopically heavy Arctic-derived moisture, and wetter climate. In the Late Holocene, sea ice expanded and regional climate became drier. This climatic transition is coincident with a documented shift in North Pacific circulation involving the Aleutian Low at ∼4 ka, suggesting a Holocene teleconnection between the North Pacific and Arctic. The HadCM3 simulations reveal that reduced sea ice leads to a strengthened Aleutian Low shifted west, potentially increasing transport of warm North Pacific water to the Arctic through the Bering Strait. Our findings demonstrate the interconnectedness of the Arctic and North Pacific on multimillennial timescales, and are consistent with future projections of less sea ice and more precipitation in Arctic Alaska.

Deep drilling reveals massive shifts in evolutionary dynamics after formation of ancient ecosystem.

The scarcity of high-resolution empirical data directly tracking diversity over time limits our understanding of speciation and extinction dynamics and the drivers of rate changes. Here, we analyze a continuous species-level fossil record of endemic diatoms from ancient Lake Ohrid, along with environmental and climate indicator time series since lake formation 1.36 million years (Ma) ago. We show that speciation and extinction rates nearly simultaneously decreased in the environmentally dynamic phase after ecosystem formation and stabilized after deep-water conditions established in Lake Ohrid. As the lake deepens, we also see a switch in the macroevolutionary trade-off, resulting in a transition from a volatile assemblage of short-lived endemic species to a stable community of long-lived species. Our results emphasize the importance of the interplay between environmental/climate change, ecosystem stability, and environmental limits to diversity for diversification processes. The study also provides a new understanding of evolutionary dynamics in long-lived ecosystems.

Human settlement of East Polynesia earlier, incremental, and coincident with prolonged South Pacific drought.

The timing of human colonization of East Polynesia, a vast area lying between Hawai'i, Rapa Nui, and New Zealand, is much debated and the underlying causes of this great migration have been enigmatic. Our study generates evidence for human dispersal into eastern Polynesia from islands to the west from around AD 900 and contemporaneous paleoclimate data from the likely source region. Lake cores from Atiu, Southern Cook Islands (SCIs) register evidence of pig and/or human occupation on a virgin landscape at this time, followed by changes in lake carbon around AD 1000 and significant anthropogenic disturbance from c. AD 1100. The broader paleoclimate context of these early voyages of exploration are derived from the Atiu lake core and complemented by additional lake cores from Samoa (directly west) and Vanuatu (southwest) and published hydroclimate proxies from the Society Islands (northeast) and Kiribati (north). Algal lipid and leaf wax biomarkers allow for comparisons of changing hydroclimate conditions across the region before, during, and after human arrival in the SCIs. The evidence indicates a prolonged drought in the likely western source region for these colonists, lasting c. 200 to 400 y, contemporaneous with the phasing of human dispersal into the Pacific. We propose that drying climate, coupled with documented social pressures and societal developments, instigated initial eastward exploration, resulting in SCI landfall(s) and return voyaging, with colonization a century or two later. This incremental settlement process likely involved the accumulation of critical maritime knowledge over several generations.

Orbital pacing and secular evolution of the Early Jurassic carbon cycle.

Global perturbations to the Early Jurassic environment (∼201 to ∼174 Ma), notably during the Triassic-Jurassic transition and Toarcian Oceanic Anoxic Event, are well studied and largely associated with volcanogenic greenhouse gas emissions released by large igneous provinces. The long-term secular evolution, timing, and pacing of changes in the Early Jurassic carbon cycle that provide context for these events are thus far poorly understood due to a lack of continuous high-resolution δ13C data. Here we present a δ13CTOC record for the uppermost Rhaetian (Triassic) to Pliensbachian (Lower Jurassic), derived from a calcareous mudstone succession of the exceptionally expanded Llanbedr (Mochras Farm) borehole, Cardigan Bay Basin, Wales, United Kingdom. Combined with existing δ13CTOC data from the Toarcian, the compilation covers the entire Lower Jurassic. The dataset reproduces large-amplitude δ13CTOC excursions (>3‰) recognized elsewhere, at the Sinemurian-Pliensbachian transition and in the lower Toarcian serpentinum zone, as well as several previously identified medium-amplitude (∼0.5 to 2‰) shifts in the Hettangian to Pliensbachian interval. In addition, multiple hitherto undiscovered isotope shifts of comparable amplitude and stratigraphic extent are recorded, demonstrating that those similar features described earlier from stratigraphically more limited sections are nonunique in a long-term context. These shifts are identified as long-eccentricity (∼405-ky) orbital cycles. Orbital tuning of the δ13CTOC record provides the basis for an astrochronological duration estimate for the Pliensbachian and Sinemurian, giving implications for the duration of the Hettangian Stage. Overall the chemostratigraphy illustrates particular sensitivity of the marine carbon cycle to long-eccentricity orbital forcing.

Effects of organic removal techniques prior to carbonate stable isotope analysis of lacustrine marls: A case study from palaeo-lake Fucino (central Italy).

RATIONALE: The suitability of organic matter (OM) removal pre-treatments in isotopic studies of lacustrine carbonates is currently under debate. Naturally occurring OM seems to have a negligible effect on the bulk isotopic composition of carbonates compared with changes induced by pre-treatments. This study provides further insights into the possible effects induced by commonly used pre-treatments on natural lacustrine carbonates. METHODS: Sixteen samples from the Fucino F1-F3 lacustrine succession (Abruzzo, central Italy) were characterised for their mineralogical and geochemical composition and each was split into three identical aliquots. One aliquot was left untreated while the remaining two were treated with NaOCl and H2 O2 dilutions. The same treatment was applied to an internal standard consisting of pure Carrara marble. The treated and untreated samples were analysed for their carbon (δ13 C values) and oxygen (δ18 O values) isotope compositions using an Analytical Precision AP2003 isotope ratio mass spectrometer. RESULTS: The samples had variable proportions of endogenic and detrital components, the detrital portion being more (less) abundant during colder (warmer) climate phases. We observed that neither the NaOCl nor the H2 O2 treatment was able to completely remove OM and therefore there was selective removal of compounds within the OM pool. A possible effect of pre-treatment is the loss of carbonates intimately interspersed within the OM, as suggested by the evolution of isotopic ratios towards the local detrital array. CONCLUSIONS: Our study highlights sample-specific changes in geochemistry associated with sample pre-treatments; however, such changes do not seem to lead to either systematic and/or predictable isotopic shifts. We suggest that the suitability of NaOCl or H2 O2 pre-treatments for OM removal should be evaluated on a case-by-case basis. In the specific case of lacustrine marls from palaeo-lake Fucino containing relatively low amounts of OM and in which both detrital and endogenic carbonates occur, both pre-treatments should be avoided.

Mediterranean winter rainfall in phase with African monsoons during the past 1.36 million years.

Mediterranean climates are characterized by strong seasonal contrasts between dry summers and wet winters. Changes in winter rainfall are critical for regional socioeconomic development, but are difficult to simulate accurately1 and reconstruct on Quaternary timescales. This is partly because regional hydroclimate records that cover multiple glacial-interglacial cycles2,3 with different orbital geometries, global ice volume and atmospheric greenhouse gas concentrations are scarce. Moreover, the underlying mechanisms of change and their persistence remain unexplored. Here we show that, over the past 1.36 million years, wet winters in the northcentral Mediterranean tend to occur with high contrasts in local, seasonal insolation and a vigorous African summer monsoon. Our proxy time series from Lake Ohrid on the Balkan Peninsula, together with a 784,000-year transient climate model hindcast, suggest that increased sea surface temperatures amplify local cyclone development and refuel North Atlantic low-pressure systems that enter the Mediterranean during phases of low continental ice volume and high concentrations of atmospheric greenhouse gases. A comparison with modern reanalysis data shows that current drivers of the amount of rainfall in the Mediterranean share some similarities to those that drive the reconstructed increases in precipitation. Our data cover multiple insolation maxima and are therefore an important benchmark for testing climate model performance.

A method for reconstructing temporal changes in vegetation functional trait composition using Holocene pollen assemblages.

Methods of reconstructing changes in plant traits over long time scales are needed to understand the impact of changing environmental conditions on ecosystem processes and services. Although Holocene pollen have been extensively used to provide records of vegetation history, few studies have adopted a functional trait approach that is pertinent to changes in ecosystem processes. Here, for woody and herbaceous fen peatland communities, we use modern pollen and vegetation data combined with pollen records from Holocene deposits to reconstruct vegetation functional dynamics. The six traits chosen (measures of leaf area-to-mass ratio and leaf nutrient content) are known to modulate species' fitness and to vary with changes in ecosystem processes. We fitted linear mixed effects models between community weighted mean (CWM) trait values of the modern pollen and vegetation to determine whether traits assigned to pollen types could be used to reconstruct traits found in the vegetation from pollen assemblages. We used relative pollen productivity (RPP) correction factors in an attempt to improve this relationship. For traits showing the best fit between modern pollen and vegetation, we applied the model to dated Holocene pollen sequences from Fenland and Romney Marsh in eastern and southern England and reconstructed temporal changes in trait composition. RPP adjustment did not improve the linear relationship between modern pollen and vegetation. Leaf nutrient traits (leaf C and N) were generally more predictable from pollen data than mass-area traits. We show that inferences about biomass accumulation and decomposition rates can be made using Holocene trait reconstructions. While it is possible to reconstruct community-level trends for some leaf traits from pollen assemblages preserved in sedimentary archives in wetlands, we show the importance of testing methods in modern systems first and encourage further development of this approach to address issues concerning the pollen-plant abundance relationship and pollen source area.

Spatial variability of 2H and 18O composition of meteoric freshwater lakes in Scotland.

Coastal regions, and in particular islands where precipitation from clouds formed out at sea occurs for the first time, are prime candidates for regions where 2H and 18O composition of precipitation will deviate significantly from the global mean geographic and physiographic trends of vapour-transport patterns. The results reported here are the outcome of a study that aimed to test this hypothesis by 'isotopographically' mapping the characteristic δ2H and δ18O signatures of Scottish freshwaters. The resulting isotope abundance landscapes or 'isoscapes' will underpin studies aiming to authenticate origin of Scottish produce but may also offer a baseline against which environmental changes could be assessed. Between April 2011 and May 2012 freshwater samples were collected from 127 different freshwater lochs and reservoirs across Scotland, and analysis results were compared to precipitation data provided by the British Geological Survey. Here we present the results of the 2H and 18O analyses of these water samples as well as the first detailed Scotland freshwater isoscapes with a grid resolution of about 5 × 5 km (0.05 degrees).

Closure of the Bering Strait caused Mid-Pleistocene Transition cooling.

The Mid-Pleistocene Transition (MPT) is characterised by cooling and lengthening glacial cycles from 600-1200 ka, thought to be driven by reductions in glacial CO2 in particular from ~900 ka onwards. Reduced high latitude upwelling, a process that retains CO2 within the deep ocean over glacials, could have aided drawdown but has so far not been constrained in either hemisphere over the MPT. Here, we find that reduced nutrient upwelling in the Bering Sea, and North Pacific Intermediate Water expansion, coincided with the MPT and became more persistent at ~900 ka. We propose reduced upwelling was controlled by expanding sea ice and North Pacific Intermediate Water formation, which may have been enhanced by closure of the Bering Strait. The regional extent of North Pacific Intermediate Water across the subarctic northwest Pacific would have contributed to lower atmospheric CO2 and global cooling during the MPT.