Changes In Ocean Circulation Research Articles

Last deglacial abrupt climate changes caused by meltwater pulses in the Labrador Sea

Freshwater perturbations are often thought to be associated with abrupt climate changes during the last deglaciation, while many uncertainties remain regarding the exact timing, pathway, mechanism, and influence of meltwater release. Here, we present very well-dated and high-resolution records from the eastern Labrador Sea representing the last 19.000 years, which demonstrate abrupt changes in sea surface characteristics. Four millennial-scale meltwater events have been identified between the last 14.000 and 8.200 years based on independent biomarker proxies and X-ray fluorescence scanning data. These events are characterized by increased sea ice formation and decreased sea surface temperatures which might have occurred within a few decades. We propose these abrupt changes were triggered by meltwater pulsing into the Labrador Sea periodically, resulting from collapse of the Laurentide-Greenland Ice Sheets caused by (sub-)surface ocean warming in the Labrador Sea. Our findings provide more precise information about impact of freshwater forcing on abrupt climate changes, which may help to improve simulations for past and future changes in ocean circulation and climate.

Contemporary oceanic radiocarbon response to ocean circulation changes

Radiocarbon (14C) is a valuable tracer of ocean circulation, owing to its natural decay over thousands of years and to its perturbation by nuclear weapons testing in the 1950s and 1960s. Previous studies have used 14C to evaluate models or to investigate past climate change. However, the relationship between ocean 14C and ocean circulation changes over the past few decades has not been explored. Here we use an Ocean-Sea-ice model (NEMO) forced with transient or fixed atmospheric reanalysis (JRA-55-do) and atmospheric 14C and CO2 boundary conditions to investigate the effect of ocean circulation trends and variability on 14C. We find that 14C/C (∆14C) variability is generally anti-correlated with potential density variability. The areas where the largest variability occurs varies by depth: in upwelling regions at the surface, at the edges of the subtropical gyres at 300 m depth, and in Antarctic Intermediate Water and North Atlantic Deep Water at 1000 m depth. We find that trends in the Atlantic Meridional Overturning Circulation may influence trends in ∆14C in the North Atlantic. In the high-variability regions the simulated variations are larger than typical ocean ∆14C measurement uncertainty of 2–5‰ suggesting that ∆14C data could provide a useful tracer of circulation changes.

Historical Changes in Wind‐Driven Ocean Circulation Can Accelerate Global Warming

AbstractMitigation and adaptation strategies for climate change depend on accurate climate projections for the coming decades. While changes in radiative heat fluxes are known to contribute to surface warming, changes to ocean circulation can also impact the rate of surface warming. Previous studies suggest that projected changes to ocean circulation reduce the rate of global warming. However, these studies consider large greenhouse gas forcing scenarios, which induce a significant buoyancy‐driven decline of the Atlantic Meridional Overturning Circulation. Here, we use a climate model to quantify the previously unknown impact of changes to wind‐driven ocean circulation on global surface warming. Wind‐driven ocean circulation changes amplify the externally forced warming rate by 17% from 1979 to 2014. Accurately simulating changes to the atmospheric circulation is key to improving near‐term climate projections.

Sea ice and productivity changes over the last glacial cycle in the Adélie Land region, East Antarctica, based on diatom assemblage variability

Abstract. Although diatoms can provide important palaeoenvironmental information about seasonal sea ice extent productivity, sea surface temperature, and ocean circulation variability, there are still relatively few studies analysing the last glacial cycle near the Antarctic continent. This study examines diatom assemblages over the last glacial cycle from core TAN1302-44, offshore Adélie Land, East Antarctica. Two distinct diatom assemblages were identified using principal component analysis (PC 1–PC 2). The PC 1 assemblage is characterised by Thalassiosira lentiginosa, Actinocyclus actinochilus, Eucampia antarctica, Azpeitia tabularis and Asteromphalus hyalinus and is associated with the interglacial, sedimentary Facies 1, suggesting that the MIS 5e and Holocene interglacials were characterised by seasonal sea ice environments with similar ocean temperature and circulation. The PC 2 assemblage is characterised by Fragilariopsis obliquecostata, Asteromphalus parvulus and Thalassiosira tumida and is associated with the glacial Facies 2. The variability of PC 2 indicates that, during the MIS 4–2 glacial and the last glaciation, there was an increase in the length of the sea ice season compared with that of the interglacial period, yet there was still no permanent sea ice cover. The initial increase of PC 2 at the start of the glaciation stage and then the gradual increase throughout late MIS 4–2 suggest that sea ice cover steadily increased, reaching a maximum towards the end of MIS 2. The increase in sea ice during glaciation and MIS 4–2 glacial is further supported by the increase in the Eucampia index (terminal/intercalary valve ratio), an additional proxy for sea ice, which coincides with increases in PC 2. Aside from the statistical results, the increase in the relative abundance of Thalassiothrix antarctica at 40 and 270 cm suggests that, during the last two deglacials, there was a period of enhanced nutrient delivery, which is inferred to reflect an increase in upwelling of Circumpolar Deep Water. Interestingly, the diatom data suggest that, during the last deglacial, the onset of increased Circumpolar Deep Water occurred after the loss of a prolonged sea ice season (decrease in PC 2) but before the ice sheet started to retreat (increase in IRD). Together, these results suggest the changes in sea ice season potentially influenced the ocean's thermohaline circulation and were important factors in driving the climate transitions. The results contribute to our understanding of the sea ice extent and ocean circulation changes proximal to East Antarctica over the last glacial cycle.

Decadal Variability of the Pacific Shallow Overturning Circulation and the Role of Local Wind Forcing

Abstract The Pacific shallow meridional overturning circulations, known as subtropical cells (STCs), link subduction in the subtropical regions to equatorial upwelling, suggesting the possibility for subtropical winds to influence equatorial sea surface temperatures (SSTs) by altering the STCs’ strength. Indeed, STC variability provides the basis for one of the mechanisms proposed to explain the origin of tropical Pacific decadal variability (TPDV). While the relationship between STC strength, as measured by their subsurface transport convergence, and equatorial SST variations is well documented, the location of the wind forcing most influential on STC variability is still being debated. In this study, we use the output of an ocean reanalysis to examine tropical Pacific Ocean surface and subsurface decadal changes during recent decades and relate them to STC variability and surface wind forcing. Our results indicate that the STC interior transport at each latitude is largely controlled by the wind forcing at that latitude rather than induced by remote subtropical wind variations. We also show that the establishment of the anomalous transport at each latitude is associated with the westward propagation of oceanic wind-forced Rossby waves, as part of the ocean adjustment process that also leads to a zonal redistribution of upper-ocean heat content at both interannual and decadal time scales. These results provide guidance for understanding the origin of TPDV by elucidating the underlying dynamics of STC variability and can have practical implications for monitoring STC variability in the tropical Pacific. Significance Statement Slow variations of the surface ocean temperature in the tropical Pacific Ocean have been shown to affect the global climate. Our study aims at better understanding the origin of these temperature anomalies by taking a closer look at the upper ocean circulation variability and its relationship with surface wind forcing. Unlike previous studies, which have related the upper ocean circulation changes to wind variations outside the tropical Pacific, we show here that the variations in upper-ocean circulation are primarily driven by local winds. This result not only clarifies which winds are most important, but also suggests a practical approach for monitoring circulation changes from surface observations.

Unradiogenic reactive phase controls the εNd of authigenic phosphates in East Antarctic margin sediment

Determining past changes in ocean circulation on the Antarctic margin is important for understanding the interactions between climate, circulation, and ice sheet retreat. However, the application of neodymium isotopes (εNd), a well-known proxy of ocean circulation, is limited on the Antarctic margin, due to the lack of carbonate preservation and inconsistency in authigenic εNd leached from bulk sediment. Here we assess the use of the εNd proxy along the continental rise of Wilkes Land, by combining analyses of seawater with phases extracted using a 10-second reductive leach in co-located surface sediments. Dissolved seawater εNd values displayed the following water mass signatures; εNd = −8.8 ± 0.2 (n = 1) for Antarctic Surface Water; εNd = −9.7 ± 0.2 (n = 2) for Winter Water; and εNd = −8.7 ± 0.3 (n = 6) for modified Circumpolar Deep Water. The sediment leachate did not reproduce a bottom water Nd signature and yielded a very variable εNd, ranging from −10.4 to −14.4. In contrast, the bulk detrital sediment εNd fell within a narrow range of −13.1 ± 0.5 (n = 29). Examination of elemental ratios and rare earth element (REE) anomalies indicates that the leaching procedure extracts exclusively authigenic phases. The strong relationship found between phosphorus (P) and REE suggests that a P-associated phase is the main REE host-phase leached, as opposed to ferromanganese oxyhydroxides. The leached europium anomalies and εNd indicate that the extracted Nd signature is influenced by two end members, one with a seawater-like εNd (∼-8) and one more negative than the bulk detritus (εNd ∼−14). Finally, the decoupling between [Nd] and εNd, as well the distinct middle-REE enrichments measured in the leach implies the authigenic εNd is controlled by diagenesis. We thus infer that the Nd signature of the leached phosphates is derived from porewater and influenced by the presence of a reactive detrital component in the sediment. Unradiogenic reactive sedimentary phases which easily dissolve into porewaters are likely sourced from the subglacial erosion of ancient metasediments and crustal remnants on the Wilkes Land coast. These new εNd data contribute to the evaluation of the εNd proxy at complex, dynamic and under-studied interfaces such as the East Antarctic margin. However, further work is still needed to develop reliable proxies of past Antarctic water masses.

Physical and biogeochemical responses in the Southern Ocean to a simple parameterization of Langmuir circulation

Langmuir circulation (LC) plays an important role in deepening the mixed layer, especially when the ocean is weakly stratified. LC parameterization has been known to improve the accuracy of the mixed-layer depth, sea surface temperature, and ocean ventilation in the Southern Ocean. However, changes in ocean dynamics and biogeochemical processes owing to LC mixing have rarely been investigated. In this study, we implemented LC parameterization to a physical–biogeochemical coupled model and examined the changes in water properties and circulation and their effects on biogeochemical processes in the Southern Ocean. The LC effect enhances the turbulent mixing length scale and turbulent kinetic energy, especially in the subantarctic region, resulting in the deepening of the mixed layer. Increased vertical mixing causes deeper warm and saline water to reach the surface layer and weakens the surface meridional velocity by transferring momentum deeper. The weakening of the meridional velocity decreases equatorward freshwater transport, causing the surface water to become saltier and denser north of 50°S adjacent to the formation site of Subantarctic Mode Water, which enhances ocean ventilation eventually. Additionally, the weak equatorward velocity drives the retreat of sea-ice south of 60°S, leading to cooler and fresher water in the southern region of 60°S. Changes in the sea-ice distribution, mixed-layer depth, and dissolved inorganic carbon distribution alter the air-sea CO2 exchange, suggesting that the LC parameterization in the model can ameliorate 10% of the air-sea CO2 exchange. The LC effects on biogeochemical tracers, such as iron (Fe) and dissolved inorganic carbon, are mainly determined by changes in ocean circulation, and the modest improvement in primary productivity is mainly attributed to the changes in the Fe distribution in the high-nutrient, low-chlorophyll region, where Fe availability limits primary production. Although the direct LC effects occur near the surface, the altered meridional circulation spreads the effects to a deeper layer and improves the overall representation of physical, biogeochemical tracers and air-sea CO2 exchange in the Southern Ocean, suggesting that a simple LC parameterization can improve the ocean model.

The Role of Ocean Circulation in Southern Ocean Heat Uptake, Transport, and Storage Response to Quadrupled CO2

Abstract In response to quadrupled CO2, the Southern Ocean primarily uptakes excess heat around 60°S, which is then redistributed by the northward ocean heat transport (OHT) and mostly stored in the ocean or released back to the atmosphere around 45°S. However, the relative roles of mean ocean circulation and ocean circulation change in the uptake and redistribution of heat in the Southern Ocean remain controversial. Here, a set of climate model experiments embedded with a novel partial coupling technique are used to separate the roles of mean ocean circulation (passive component) and ocean circulation change (active component). For the ocean heat uptake (OHU) response, the mean ocean circulation and ocean circulation change are of equal importance. The OHT response south of 50°S is mainly determined by mean ocean circulation, while the ocean circulation change generates an anomalous southward OHT north of 50°S. A heat budget analysis finds that the divergence of passive OHT acts to balance the passive surface heat gain to the south of ∼50°S, while the convergence of active OHT acts to balance the active surface heat loss to the north of ∼50°S. Intriguingly, all the increase in ocean heat storage (OHS) is attributable to the passive component, with the ocean circulation change playing almost no role. In the Southern Ocean, both the active and the passive ocean heat transports are overcompensated by the reverse atmospheric heat transport via the Bjerknes compensation.

Late Eocene signals of oncoming Icehouse conditions and changing ocean circulation, Antarctica

The end of the Eocene greenhouse world was the most dramatic phase in the long-term Cenozoic cooling trend. Here we use 75,000 km of multi/single channel seismic reflection data from offshore Prydz Bay, Antarctica, to provide new insight on the Paleogene stratigraphic transition from greenhouse to icehouse conditions and reorganizing the ocean circulation changes that were invigorated by the cooling and glaciation. We identify a new prominent Paleogene transitional phase (Greenhouse to Icehouse) preserved in the deep-water sedimentary record by correlating from shelf to the continental slope. The occurrence of mega-Mass Transport Deposits (MTDs) on the slope during an early stage in the transition suggests significant instability and collapse of the upper part of the continental margin. A second stage of the transition is represented by the growth of a well-defined set of continental slope clinoforms. We estimate the formation age of the MTDs and clinoforms to be around Eocene-Oligocene Transition. The formation of the clinoforms in the transitional phase indicates sea level has risen, and large volumes sediment delivered to the margin by marine-terminating glaciers on the shelf. Finally, a subsequent marked migration of the margin depocenter toward the west and northwest, attests the onset of drift sedimentation and full glacial conditions, suggesting a more vigorous ocean circulation as the Earth entered the icehouse conditions after the Eocene-Oligocene boundary.

The calcareous nannofossil record of the uppermost Maastrichtian-lower Palaeocene in the Kırıkkale Basin, in the Central Anatolian Region (Turkey)

The Late Cretaceous–Early Paleogene (K–Pg) was a critical period of transition in geological time. This period encompassed short-term climatic fluctuations on a global scale, changes in ocean circulation, and sudden and large extinctions of marine and terrestrial organisms. In the study area, located in the mid to low latitudes, the Late Cretaceous and Early Paleogene were very tectonically active due to the positioning of the site close to the collision zone of two large continents. The impacts of the global K–Pg crisis can be observed in the study area. In this study, the calcareous nannofossil contents of late Maastrichtian–Danian sediments were studied, and the nannofossil biostratigraphy determined, from samples from the Samanlık and Dizilitaşlar Formations, deposited in the Kırıkkale Basin. From three stratigraphic sections, 26 nannofossil genera and 36 nannofossil species were identified from the Late Maastrichtian UC20aTP and UC20bTP biozones and the NP1 and NP2 biozones of the Danian. Additionally, it was determined that the K–Pg boundary was not continuous in the study area. In the Kırıkkale Basin, relatively low abundances of Micula decussate Vekshina, 1959 signals a diagenetic effect and stressful environment in the Late Maastrichtian, whereas the relatively low abundances of Thoracosphaera operculata Bramlette & Martini, 1964, Braarudosphaera bigelowii (Gran & Braarud, 1935) Deflandre, 1947 and Futyania petalosa (Ellis & Lohmann, 1973) Varol, 1989 in the Danian assemblages indicate unstable environmental conditions and major environmental perturbations that reflect tectonic activity in the region. No nannofossils were encountered in those samples taken from turbiditic levels, which contained high proportions of sand.

Glacial‐Interglacial Controls on Ocean Circulation and Temperature During the Permo‐Carboniferous

AbstractThe apex of Earth's penultimate icehouse during the Permo‐Carboniferous coincided with dramatic glacial‐interglacial fluctuations in atmospheric CO2, sea level, and high‐latitude ice. Global transformations in marine fauna also occurred during this interval, including a rise to peak foraminiferal diversity, suggesting that glacial‐interglacial climate change impacted marine ecosystems. Nevertheless, changes in ocean circulation and temperature over the Permo‐Carboniferous and their influence on marine ecosystem change are largely unknown. Here, we present simulations of glacial and interglacial phases of the latest Carboniferous‐early Permian (∼305‐295 Ma) using the Community Earth System Model version 1.2 to provide estimates of global ocean circulation and temperature during this interval. We characterize general patterns of glacial and interglacial surface ocean currents, temperature, and salinity, and compare them to the documented abundance and distribution of Permo‐Carboniferous marine fauna as well as a preindustrial climate simulation. We then explore how glacial‐interglacial changes in atmospheric CO2, sea level, and high‐latitude ice extent impact thermohaline circulation. We find that glacial‐interglacial changes in equatorial surface temperatures are consistently ∼3–6°C. Ocean circulation is stronger overall in the glacial simulation, particularly as lower atmospheric CO2 enables deep convection in the Northern Hemisphere. Wind‐driven circulation, heat transport, and upwelling intensity are stronger overall in the Permo‐Carboniferous superocean relative to the preindustrial oceans at the same level of atmospheric CO2. We also find that CO2‐induced glacial conditions of the early Permian may have promoted foraminiferal diversity through increased thermal gradients and suppressed riverine input in marine shelf environments.

Active Nordic Seas deep-water formation during the last glacial maximum

The Nordic Seas are the primary location where the warm waters of the North Atlantic Current densify to form North Atlantic Deep Water, which plays a key part in the modern Atlantic Meridional Overturning Circulation. The formation of dense water in the Nordic Seas and Arctic Ocean and resulting ocean circulation changes were probably driven by and contributed to the regional and global climate of the last glacial maximum (LGM). Here we map the source and degree of mixing of deep water in the Nordic Seas and through the Arctic Gateway (Yermak Plateau) over the past 35 thousand years using neodymium isotopes (εNd) measured on authigenic phases in deep-sea sediments with a high spatial and temporal resolution. We find that a large-scale reorganization of deep-water formation in the Nordic Seas took place between the LGM (23–18 thousand years ago) and the rapid climate shift that accompanied the subsequent deglaciation (18–10 thousand years ago). We show that homogeneous εNd signatures across a wide range of sites support LGM deep-water formation in the Nordic Seas. In contrast, during the deglaciation, disparate and spatially variable εNd values are observed leading to the conclusion that deep-water formation may have been reduced during this time.

A modeling framework to understand historical and projected ocean climate change in large coupled ensembles

Abstract. The ocean responds to climate change through modifications of heat, freshwater and momentum fluxes at its boundaries. Disentangling the specific role of each of these contributors in shaping the changes of the thermohaline structure of the ocean is central for our process understanding of climate change and requires the design of specific numerical experiments. While it has been partly addressed by modeling studies using idealized CO2 forcings, the time evolution of these individual contributions during historical and projected climate change is however lacking. Here, we propose a novel modeling framework to isolate these contributions in coupled climate models for which large ensembles of historical and scenario simulations are available. The first step consists in reproducing a coupled pre-industrial control simulation with an ocean-only configuration, forced by prescribed fluxes at its interface, diagnosed from the coupled model. In a second step, we extract the external forcing perturbations from the historical+scenario ensemble of coupled simulations, and we add them to the prescribed fluxes of the ocean-only configuration. We then successfully replicate the ocean's response to historical and projected climate change in the coupled model during 1850–2100. In a third step, this full response is decomposed in sensitivity experiments in which the forcing perturbations are applied individually to the heat, freshwater and momentum fluxes. Passive tracers of temperature and salinity are implemented to discriminate the addition of heat and freshwater flux anomalies from the redistribution of pre-industrial heat and salt content in response to ocean circulation changes. Here, we first present this general framework and then apply it to the IPSL-CM6A-LR model and its ocean component NEMO3.6. This framework brings new opportunities to precisely explore the mechanisms driving historical and projected ocean changes within single climate models.

Assessing neodymium isotopes as an ocean circulation tracer in the Southwest Atlantic

The global overturning ocean circulation plays a key role in global climate by distributing heat around the Earth and by triggering or amplifying major climate changes. Neodymium (Nd) isotopes are widely used to trace present-day and past ocean circulation changes; however, their value as a circulation tracer has been increasingly challenged by studies that have focused on processes that can modify seawater Nd isotope ratios (for example, seawater interaction with particles, pore water fluxes from sediments). The Southwest Atlantic represents an excellent test bed for investigating the integrity of Nd isotopes as an ocean circulation tracer, as it includes the major Atlantic northern and southern hemisphere-sourced end-member water masses associated with the global overturning ocean circulation, Antarctic Intermediate Water, North Atlantic Deep Water, and Antarctic Bottom Water (AAIW, NADW, and AABW, respectively) and potential regional sources of Nd that could impact that integrity. This study reports Nd isotope data on the GEOTRACES GA02 Southwest Atlantic Meridional Transect, spanning the equator to ∼50°S. Below the pycnocline, substantial non-conservative behavior is observed only in samples dominated by AAIW in the northern portion of the transect (∼25°S to the equator); this appears to reflect addition of Nd from the cratons of South America or Africa from above the pycnocline. This effect is not observed at depths directly below dominated by NADW. Otherwise, Nd isotopes behave as a near-conservative water mass tracer along the transect, with 48% of samples within analytical error of the predicted value from water mass mixing, and 84% within 0.9 εNd-units (∼3 times the analytical error), thus confirming its potential at most depths and locations in the Southwest Atlantic to reconstruct past ocean circulation changes.

Ocean circulation and climate variability in the northern South China Sea during the Greek Minimum derived from coral Δ14C and Sr/Ca records

Ocean circulation transports heat, salt and nutrients, and has profound impacts on the marine environment and climate change. However, the seasonal to centennial variations of the paleocirculation of the South China Sea (SCS) and their driving forces are still unclear due to limited records. Here we reconstructed a high-resolution Δ14C record of a coral from Sanya to investigate the seasonal variations of the SCS ocean circulation around 2300 cal yr BP during a grand solar minimum (GSM) period. The seasonal Δ14C variability shows the influence of coastal upwelling caused by the East Asian Summer Monsoon (EASM) in summer and the effect of water intrusion from the western Pacific Ocean driven by the Kuroshio Intrusion (KI) in winter. We compiled the marine radiocarbon reservoir correction (ΔR) records since 2500 cal yr BP, which indicate a gradual decrease of upwelling significantly correlated with the EASM. Comparisons with other climate records suggest that both the EASM and KI may regulate the ocean circulation variability on centennial time scales. As a good index of the sea surface temperature (SST), our Sr/Ca record and spectral analysis results show a low temperature and a low frequency of El Niño-Southern Oscillation (ENSO) events around 2300 cal yr BP. Moreover, the composite coral Sr/Ca-SST and ENSO index records since 2500 cal yr BP show relatively low SST and weak ENSO during GSMs, supporting the modulation of tropical SST and ENSO by solar activity. Our study has provided high-resolution proxy data and revealed the driving forces of ocean circulation and climate change in the SCS at multiple time scales, which should be considered in further modeling work.

Decadal Weakening of Abyssal South China Sea Circulation

AbstractThe global ocean mass has significantly changed during recent decades, which may induce changes in the abyssal ocean circulation. Due to the lack of in situ observations, the response of the abyssal South China Sea (SCS) circulation to ocean mass changes remains unclear. The ocean state estimate, Estimating the Circulation and Climate of the Ocean (ECCO), provides the long‐term ocean mass changes in terms of ocean bottom pressure (OBP). Here we use the ECCO OBP data to quantify decadal changes in the abyssal SCS circulation and reveal a weakening trend. The OBP gradient trend manifests in forms of an anomalous anticyclonic circulation that is intensified by topographic effects along the continental slopes to reduce the existing abyssal circulation. The weakening abyssal circulation corresponds to the weakening Luzon Strait overflow that yields a decreasing vorticity budget in the abyssal SCS.

The Quantitative Analysis of Water Mass during Winter on the East China Sea Shelf Using an Extended OMP Analysis

The distribution and quantification of water masses on the East China Sea (ECS) shelf is important for identifying and understanding historical climate-driven changes in ocean properties and circulation in the region. We applied an extended Optimum Multiparameter (eOMP) analysis to quantify the relative contribution of water masses using wintertime temperature, salinity, nitrate (NO3−), phosphate (PO43−), and silicate (SiO32−) measurements from a five-cruises dataset spanning from 2013 to 2018. Average ratios (NO3−:PO43−:SiO32− = 47:1:35) derived from field observations were used to correct the equations referring to the chemical parameters. Our analysis indicated that wintertime seawater on the ECS shelf consisted mainly of Changjiang Dilute Water (CDW), Yellow Sea Coastal Water (YSCW), Taiwan Warm Current Water (TWCW), and East China Sea Shelf Water (ECSSW). The results from the eOMP analysis demonstrated the natural boundaries of four water masses during winter. The interannual variability of water masses showed that the CDW distribution was relatively stable in winter, and there was strong anticorrelation between the YSCW and TWCW extents, suggesting that these two water masses mostly displace each other in the north-south direction.

Eurasian Ice Sheet derived meltwater pulses and their role in driving atmospheric dust activity: Late Quaternary loess sources in SE England

The role of Quaternary ice sheet fluctuations in driving meltwater pulses and ocean circulation perturbations is widely acknowledged. What is less clear is the role of these processes in driving changes in past atmospheric dust activity, and possible wider links between dust and climate. Terrestrial windblown dust (loess) deposits along the northern fringe of the European loess belt potentially record past atmospheric dust emission from regions close to the former Eurasian Ice Sheet (EIS) and provide a means to evaluate the role of ice sheet fluctuations in the past dust cycle. Numerical loess chronologies across this region generally agree on greatly enhanced dust deposition rates during MIS 2, when the EIS reached its maximum extent. Yet, uncertainties over the sources of this material prevent understanding of the precise causes of this greatly enhanced atmospheric dustiness, and any potential link to ice sheet fluctuations and climate. In southeast England, loess accumulation dominantly occurred in two phases centered on 25–23.5 ka and 20–19 ka, matching the timing of coalescence of the Fennoscandian and British-Irish ice sheets and specifically advances and retreats of nearby ice lobes in the western North Sea. As such, these deposits provide an ideal test of the role of ice sheet fluctuations in atmospheric dust dynamics. Here we undertake such a test through a detailed provenance study of loess in southeast England and potential dust source sediments across the North Sea region. We group extensive new and published data sets of detrital zircon U–Pb ages from basement rocks and Cenozoic sediments in the North Sea area, which not only provide new insight into both loess source, but also the nature of sediment transport over NW Europe into the North Sea basin more widely. Multi-proxy evidence allows us to unambiguously identify ice sheet derived sediments in the exposed North Sea basin as the dominant source of loess in southeast England, while fluvial sediments delivered by rivers draining Continental Europe possibly contributed additional source material to the first loess accumulation phase. We propose that sudden retreats of the North Sea Lobe released substantial amounts of sediment rich meltwater into the southern North Sea and Channel basins, driving accelerated dust emission, loess deposition and provenance variability in NW Europe during MIS 2. Moreover, we propose that this model of dust activity driven by proglacial sediment availability may be applicable for EIS marginal regions more widely, even where resultant loess cover is rarely preserved due to extensive erosion and reworking along the ice marginal spillway. This implies the role of ice sheets in controlling wider dust emission may be underestimated. In addition to driving changes in ocean circulation through meltwater pulses, ice sheet dynamics in the Quaternary may have also driven substantial and abrupt changes in atmospheric dust activity. This mechanism may in part explain the coupling between dust and climate events widely seen in Quaternary dust sediment records and suggests a possible major role of high latitude dust emission in MIS 2 dustiness across Europe and beyond.

High resolution benthic foraminiferal δ18O and δ13C records at ODP site 807 over the past 5 Ma, Ontong Java Plateau: Evolution of North Pacific ventilation, Pliocene to Holocene

The western equatorial Pacific is pivotal to precisely understand the global climate change and its evolution over geological time. However, continuous paleoceanographic records with high time resolution and accurate age model over the last 5 Myr are few in this important ocean region. Here, we present a high time resolution (2–3 kyr) benthic foraminiferal δ 18 O and δ 13 C record from Ocean Drilling Program Site 807 on Ontong Java Plateau in the western equatorial Pacific and establish an accurate age model for the past 5.025 Myr. Comparison of the benthic δ 13 C records from the west and east equatorial Pacific, and the North and South Pacific indicates that water aging effects have dominated since the Pliocene. From 4.0 Ma to 2.7 Ma, a rapid decrease in benthic δ 13 C gradients (Δδ 13 C W-E ) between the west (Hole 807A) and east (ODP Site 849 and IODP Site U1338) equatorial Pacific potentially resulted from strong ventilation in the North Pacific during the warm Pliocene. The subsequent gentle decrease in benthic δ 13 C gradients might indicate stratification in the North Pacific and reduction of biogenic flux with the intensification of the Northern Hemisphere Glaciation (NHG) at 2.7 Ma. Both the Δδ 18 O W-E and Δδ 13 C W-E are strongly paced by 405 ka long eccentricity cycles and 41 kyr obliquity cycle, revealing the complex interplay of both high and low latitude climatic processes on deep Pacific circulation changes. • Continuous 5 Myr long high time resolution benthic foraminiferal δ 18 O and δ 13 C records in the western equatorial Pacific. • West-east equatorial Pacific δ 13 C gradient indicates the evolution of the Pacific deep ocean circulation over the past 5 Myr. • 405-kyr eccentricity cycle modulates the Pacific deep ocean circulation changes.

The Importance of Spatial and Temporal Scales in Understanding Chlorophyll Variability in the Southern Ocean

Polar regions are undergoing dramatic, rapid, and possibly irreversible changes. Substantial shifts in patterns of sea ice extent and thickness have cascading effects on polar ecosystems (including phytoplankton), with implications for carbon cycling and global climate. Phytoplankton growth is closely tied to environmental variables such as light and nutrient availability, which are sensitive to climate-induced changes in upper ocean circulation, stratification, and sea ice cover. Recently, Prend et al. (2022, https://doi.org/10.1029/2022GB007329) investigated temporal and spatial scales of chlorophyll (a proxy for phytoplankton biomass) variability in the Southern Ocean. They demonstrated that the dominant temporal scale of variability is sub-seasonal (∼0.5–3 months). The implications of this are two-fold: first, climate oscillations (such as the Southern Annular Mode) are not major drivers of year-to-year variation in chlorophyll; second, intermittent bursts of chlorophyll, generated by small-scale processes such as storms and eddies, dictate the annual mean chlorophyll concentration. Additionally, spatial autocorrelation for chlorophyll concentration varied by time scale: seasonal chlorophyll variability was correlated over much larger areas than were variations in year-to-year chlorophyll concentration. Based on Prend et al. (2022, https://doi.org/10.1029/2022GB007329), future work should be cognizant of (a) the spatio-temporal scales over which chlorophyll is averaged and (b) the need to focus on small-scale, sub-seasonal events (rather than large-scale climate oscillations) to mechanistically explain chlorophyll variability.