Intermediate Water Circulation Research Articles

Dissolved Iron Concentration and the Solubility Inferred by Humic‐Like Fluorescent Dissolved Organic Matter in the Intermediate Water in the North Pacific Including the Marginal Seas

AbstractThe marginal seas have been found to be important external sources of dissolved iron (Fe) in the North Pacific through circulation of intermediate water. Here, we show comprehensive spatial distributions of dissolved Fe concentrations and Fe(III) solubilities inferred from humic‐like fluorescent dissolved organic matter (FDOMH) over the North Pacific, including the marginal seas, the Sea of Okhotsk, and the Bering Sea. FDOMH was used as a proxy of chemical speciation of dissolved Fe in the intermediate and deep waters where liner relationships were previously observed between FDOMH and Fe(III) solubility. When dissolved Fe concentration exceeds Fe(III) solubility, Fe(III) solubility is assumed to be equivalent to concentration of FDOMH‐Fe complexes, and excess dissolved Fe concentration above the Fe(III) solubility is assumed to be colloidal Fe which is not complexed with FDOMH. In the intermediate water, the dissolved Fe concentration exceeded the Fe(III) solubility in the marginal seas, while excess Fe(III) solubility was evident downstream of the intermediate water circulation, suggesting that the major dissolved Fe chemical form derived from shelf and slope sediments in the marginal seas changed from colloidal Fe to FDOMH‐Fe complexes. With the previous findings, namely the dominance of labile particulate Fe in total Fe in the intermediate water of the Sea of Okhotsk and the Bering Sea, we hypothesized that the average size of sediment‐derived Fe decreases during transportation by intermediate water, most likely due to reversible scavenging with the highest removal rate for labile particulate Fe and the lowest removal rate for FDOMH‐Fe complexes.

Intermediate water circulation drives distribution of Pliocene Oxygen Minimum Zones

Oxygen minimum zones (OMZs) play a critical role in global biogeochemical cycling and act as barriers to dispersal for marine organisms. OMZs are currently expanding and intensifying with climate change, however past distributions of OMZs are relatively unknown. Here we present evidence for widespread pelagic OMZs during the Pliocene (5.3-2.6 Ma), the most recent epoch with atmospheric CO2 analogous to modern (~400-450 ppm). The global distribution of OMZ-affiliated planktic foraminifer, Globorotaloides hexagonus, and Earth System and Species Distribution Models show that the Indian Ocean, Eastern Equatorial Pacific, eastern South Pacific, and eastern North Atlantic all supported OMZs in the Pliocene, as today. By contrast, low-oxygen waters were reduced in the North Pacific and expanded in the North Atlantic in the Pliocene. This spatially explicit perspective reveals that a warmer world can support both regionally expanded and contracted OMZs, with intermediate water circulation as a key driver.

The first appearance of Glacial North Atlantic Intermediate Water (GNAIW) during the Mid-Pleistocene transition

The Mid-Pleistocene Transition (MPT) is arguably the most prominent shift in the Earth's climate system during the entire Pleistocene. It is characterized by a shift from a ∼ 40 kyr to a ∼ 100 kyr glacial cyclicity, going in hand with considerable growth of glacial continental ice-sheets. In search for potential drivers of these changes in the cryosphere, a slow-down of deep ocean circulation has been proposed as an important factor that might have reduced the atmospheric pCO2 level and thus created boundary conditions favourable for pertaining large ice-sheets. Additionally, as argued for the Last Glacial Maximum, enhanced production of intermediate waters might enhance intermediate to deep oceanic stratification and thus suppress CO2 release from the deep ocean and thereby reduce atmospheric greenhouse gas concentrations. To investigate potential changes in the North Atlantic intermediate water-mass configuration across the MPT we utilize δ18O and δ13C records obtained on the deep thermocline-dwelling foraminifera Globorotalia crassaformis from mid-latitude International Ocean Discovery Program (IODP) Site U1313. In conjunction with benthic and planktic δ18O and δ13C data from reference sites for Labrador Sea Water (IODP Site U1305) and intermediate water formation in the north-eastern North Atlantic (Ocean Drilling Program Site 982) our data show that major changes in the production and dispersal of intermediate water masses took place across the MPT. In particular, we note a shoaling of glacial intermediate waters commencing with Marine Isotope Stage (MIS) 22, which we interpret as the first occurrence of Glacial North Atlantic Intermediate Water, a prominent constituent of Late Pleistocene glacial water masses in the Atlantic Ocean. We further infer that the amount and vertical position of the warm and saline Mediterranean Outflow Water (MOW) had a major impact on the spatial configuration of North Atlantic intermediate water masses during the MPT, due to its capacity to modulate subsurface heat transport and isopycnal configuration. Our new findings thus provide evidence for a so far underestimated role of intermediate water circulation in shaping oceanic and potentially atmospheric changes during the MPT.

Distributions of aluminum, manganese, cobalt, and lead in the western South Pacific: Interplay between the South and North Pacific

Aluminum (Al), manganese (Mn), cobalt (Co), and lead (Pb) are strongly scavenged from seawater. We reported that each element is uniquely related to ocean circulation in the North Pacific (Zheng et al., 2019). Herein, we present the full-depth distributions of these elements in the western South Pacific, which include meridional sections along 170°W (GEOTRACES GP19). We determined dissolved (d) and total dissolvable (td) concentrations using filtered and unfiltered seawater without UV treatment, and we calculated labile particulate (lp) concentrations as the difference between td and d concentrations. This and the previous studies present the basin scale distributions, which enable us to investigate first order processes that drive the biogeochemistry of Al, Mn, Co, and Pb in the Pacific Ocean. The meridional section of dAl along 170°W (GP19)-160°W (GPc06) from 64°S to 54°N indicates that elevated concentrations (maximum 6.1 nmol/kg) occur between 40°S and 10°S from surface to bottom. However, the maxima of lpAl occur at high latitudes. The lpAl/tdAl ratio has a minimum of 0.26 ± 0.12 (ave ± sd, n = 116) in the zone from 30°S to 0°S. Based on these results, we propose a hypothesis that weathering on land has a significant effect on the distribution of Al in the ocean. Intensive weathering on tropical and subtropical islands and Australia forms kaolinite-dominated soils and laterite. This process provides dAl and kaolinite to the ocean. The supply of kaolinite results in kaolinite-dominated sediments that become a major bottom source for dAl. In contrast, strong sources of Mn and Co are continental shelves around the northern boundary. Dissolved Mn and dCo are released from sediments by manganese reduction and carried by intermediate water circulation. In particular, dCo spreads in the North Pacific Intermediate Water (NPIW), Equatorial Pacific Intermediate Water (EqPIW), and Antarctic Intermediate Water (AAIW); 23–59 pmol/kg at a potential density anomaly (σθ) of 27.0. This is partly owing to the uptake of dCo by phytoplankton and remineralization from settling particles. The dPb concentrations are 10 ± 6 pmol/kg (n = 397) in the South Pacific and 30 ± 20 pmol/kg (n = 566) in the North Pacific. The distribution of dPb is characterized by a maximum in the Subtropical Mode Water (SMW) and Central Mode Water (CMW) with σθ∼26 in the North Pacific. These results indicate that Pb is predominantly supplied by anthropogenic aerosols from Asia and Russia to the Pacific Ocean. Because Pb is not actively taken up by phytoplankton, Pb enters mode waters during winter convection and is transported with mode waters.

A 300 000-year record of cold-water coral mound build-up at the East Melilla Coral Province (SE Alboran Sea, western Mediterranean)

Abstract. This study provides a detailed reconstruction of cold-water coral mound build-up within the East Melilla Coral Province (southeastern Alboran Sea), more precisely at the northern part of Brittlestar Ridge I, over the last 300 kyr. The multiproxy investigation of core MD13-3462G reveals that mound build-up took place during both interglacial and glacial periods at average aggradation rates ranging between 1 and 10 cm kyr−1. These observations imply that corals never thrived but rather developed under stressful environmental conditions. Maximum aggradation rates of 18 cm kyr−1 are recorded during the last glacial period, hence providing the first evidence of coral mound development during this time period in the western Mediterranean. The planktonic (Globigerina bulloides) and benthic (Lobatula lobatula) δ18O records from core MD13-3462G show typical interglacial–glacial variations during the last two interglacial–glacial cycles. This is in contrast with δ18O records generally recovered from coral mounds and highlights that the northern part of Brittlestar Ridge I experienced reduced albeit relatively continuous accretion. High abundances of infaunal benthic foraminifera (Bulimina marginata, Bulimina striata, and Uvigerina mediterranea) suggest that weak seafloor oxygenation associated with important terrestrial organic matter input characterized interglacial periods, whilst the dominance of large epibenthic species (Discanomalina coronata and Lobatula lobatula) and Miliolids is probably linked to stronger Levantine Intermediate Water circulation and fresher organic matter input during glacial periods. In addition, the computed tomography (CT) quantification of macrofaunal remains shows that the bryozoan Buskea dichotoma is present throughout the entire 300 kyr of mound build-up history, with the exception of MIS 5, and is possibly a key contributor to mound development during glacial periods. The comparison of our observations to other long-term coral mound records demonstrates that western and central Mediterranean coral mounds do not show concurrent build-up over interglacial–glacial cycles, implying that their development may be driven by regional and local environmental forcing.

A new assessment of the circulation of Atlantic and Intermediate Waters in the Eastern Mediterranean

A simulation of the Mediterranean circulation between 2011 and 2020 at a resolution of 3–4 km in the Eastern basin was compared to vertical profiles and horizontal distributions of temperature and salinity from Argo profilers distributed throughout the basin. The comparison is marked by high temporal (∼0.9) and spatial (0.6–0.8) correlations and low biases. Comparisons of SST with satellite imagery have also shown strong similarities for numerous structures over a wide range of spatial scales. The simulation is used to describe the mean circulation of surface Atlantic Waters and Intermediate Waters in winter and summer.The surface circulation is cyclonic alongslope, stronger and more stable in winter. In summer, the current veins are sometimes interrupted and replaced by trains of eddies like in the South Ionian. In other cases, the current becomes very narrow and stuck to the coast as along the Ionian east coast or the Middle East coast. In winter, surface and Levantine Intermediate Waters exit from the Levantine mainly through the Aegean, while in summer, they exit westward south of Crete. The Aegean tends in summer to be isolated by eddies that develop on both sides of the Cretan Arc. The juxtaposition of Ierapetra, the Rhodes Gyre and the Mersa-Matruh Eddies produces a southward path across the Levantine basin at about 27 − 28°E which delimits a large cyclonic circulation to the east which tends to separate the two parts of the basin (west and east Levantine). Concerning the Levantine Intermediate Waters, the alongslope cyclonic circulation all around the Levantine basin in winter is no longer maintained in summer as a large anticyclonic circulation occupies the southeast of the basin. The intermediate waters entering the Ionian either through southern Crete in summer or through the Aegean in winter, are submitted to a strong northward, southward and even westward dispersion by Pelops and the nearby anticyclonic areas. The presence of recurrent anticyclones between 35 and 37°N along a band extending from west to east of the Ionian also produces a vertical dispersion of intermediate water. Finally, the circulation of intermediate water in the South Ionian is marked by an important seasonality with the presence in summer of a large anticyclonic circulation that seems to be wind induced and finally drives a secondary branch to the Sicily Channel. A climatology for the transport through the different straits is discussed and simplified representations of the circulation are proposed.

On the Circulation and Thermohaline Properties of the Eastern Mediterranean Sea

The circulation of the Eastern Mediterranean Sea is characterized by numerous recurrent or permanent anticyclonic structures, which modulate the pathway of the main currents and the exchange of the water masses in the basin. This work aims to describe the main circulation structures and thermohaline properties of the Eastern Mediterranean with particular focus on two anticyclones, the Pelops and the Cyprus gyres, using in-situ (drifters and Argo floats) and satellite (altimetry) data. The Pelops gyre is involved in the circulation and exchange of Levantine origin surface and intermediate waters and in their flow toward the Ionian and the Adriatic Sea. The Cyprus Gyre presents a marked interannual variability related to the presence/absence of waters of Atlantic origin in its interior. These anticyclones are characterized by double diffusive instability and winter mixing phenomena driven by salty surface waters of Levantine origin. Conditions for the salt finger regime occur steadily and dominantly within the Eastern Mediterranean anticyclones. The winter mixing is usually observed in December–January, characterized by instability conditions in the water column, a gradual deepening of the mixed layer depth and the consequent downward doming of the isohalines. The mixing generally involves the first 200 m of the water column (but occasionally can affect also the intermediate layer) forming a water mass with well-defined thermohaline characteristics. Conditions for salt fingers also occur during mixing events in the layer below the mixed layer.

A study of intermediate water circulation in the Strait of Georgia using tracer-based, Eulerian, and Lagrangian methods

AbstractThe intermediate circulation of the Strait of Georgia, British Columbia, Canada, plays a key role in dispersing contaminants throughout the Salish Sea, yet little is known about its dynamics. Here, we use hydrographic observations and hindcast fields from a regional 3D model to approach the intermediate circulation from three perspectives. Firstly, we derive and model a “seasonality” tracer from temperature observations to age the water, estimate mixing, and infer circulation. Secondly, we analyze modeled velocity fields to create mean current maps and examine the advective and diffusive components of the mean flow field. Lastly, we calculate Lagrangian trajectories to derive Transit Time Distributions and Lagrangian statistics. In combination, these analyses provide an overview of the mean intermediate circulation that can be summarized as follows: subducting water in Haro Strait ventilates the intermediate water primarily via an up-strait boundary current that flows along the eastern shores of the southernmost basin in 1–2 months. This inflowing water is either incorporated into the interior of the basin, recirculated southwards, or transported into the northernmost basin, mixing steadily with adjacent water masses during its transit. A second, shallower ventilating jet emanates southwards from Discovery Passage, locally modifying the Haro Strait inflow signal. Outside of these well-defined advective features, diffusive transport dominates in the majority of the region. The intermediate renewal signal fully ventilates the region in 100–140 days, which serves as a benchmark for contaminant dispersal timescale estimates.

Widespread distribution of allochthonous fluorescent dissolved organic matter in the intermediate water of the North Pacific

Visible wavelength fluorescent dissolved organic matter (FDOM), usually defined as humic-like FDOM, plays important roles in marine carbon and iron cycles as biorefractory dissolved organic matter and organic ligands, respectively. The major fractions of FDOM in the open ocean have been considered to be of marine origin (autochthonous) according to linear relationships between FDOM and apparent oxygen utilization (AOU) in the mesopelagic (200–1000 m) and bathypelagic (>1000 m) layers. Recently, Yamashita et al. (2020) quantified allochthonous FDOM from the positive deviation of the general FDOM–AOU relationship in the bathypelagic layer and found that allochthonous FDOM derived from the Sea of Okhotsk was conservatively transported to the western North Pacific through the circulation of intermediate water, including North Pacific Intermediate Water (NPIW). However, the contribution of allochthonous FDOM from the Bering Sea, the other source region of NPIW, has not been evaluated. Here, we determined the distributions of allochthonous FDOM in the entire North Pacific, including the Sea of Okhotsk and the Bering Sea, and the spatial distribution of allochthonous FDOM in the density range of NPIW for the entire North Pacific. We found that the major source region of allochthonous FDOM in NPIW was the Sea of Okhotsk but not the Bering Sea, although the water mass contributing to the lower part of NPIW is known to be mainly derived from the Bering Sea. Such different contributions of allochthonous FDOM from the marginal seas are likely due to a lack of expansion of the dense shelf water, which forms at coastal polynya and interacts with shelf sediments with a strong tidal current, to the basin region of the Bering Sea. We also demonstrated that allochthonous FDOM derived from the shelf sediments of the Sea of Okhotsk was conservatively distributed to the wide area of the North Pacific through the circulation of intermediate water, particularly the upper part of NPIW. Furthermore, the negative deviations of the FDOM-AOU relationship in the bathypelagic layer were evident for other water masses, such as Antarctic Intermediate Water (AAIW), suggesting that FDOM can help determine the spatiotemporal distribution of NPIW and its mixing with other water masses, such as AAIW.

Shelf humic substances as carriers for basin-scale iron transport in the North Pacific

Iron is one of the key elements controlling phytoplankton growth in large areas of the global ocean. Aeolian dust has traditionally been considered the major external source of iron in the North Pacific. Recent studies have indicated that sedimentary iron from the shelf region of the Sea of Okhotsk has a strong impact on the iron distribution in the North Pacific, while the mechanism supporting its long-distance transport remains poorly understood. Here, we report that refractory shelf humic substances, which complex and carry dissolved iron, are transported conservatively at least 4000 km from the shallow sediments of the Sea of Okhotsk to the subtropical North Pacific with the circulation of intermediate water. This result indicates that shelf humic substances are probably one of the key factors shaping the distribution of dissolved iron in the ocean interior.

Seismic Evidences of Contourites on the Rio Grande Rise, Southwest Atlantic

Several features probably formed by contour bottom currents were recognized on the Rio Grande Rise by high-resolution seismic survey in the transatlantic profiles of the #32 and #52 cruises of R/V Akademik Ioffe carried out by the Shirshov Institute of Oceanology, Russian Academy of Sciences. The deposits of mixed turbidite–contourite sedimentary systems near the base of its southern and northern slopes seem to be formed as a result of the interaction of downslope gravity flows and the along-slope deep-water Lower Circumpolar Water + Weddell Sea Deep Water current. Sediment waves and disputable small plaster contourite drifts are probably formed by circulation of the Antarctic Intermediate Water in the Cruzeiro-do-Sultrough at the top of the rise.

Changes in western Mediterranean thermohaline circulation in association with a deglacial Organic Rich Layer formation in the Alboran Sea

The accumulation of an Organic Rich Layer (ORL) during the last deglaciation in the Alboran Sea (western Mediterranean Sea) and its link to changes in deep and intermediate water circulation are here investigated. Benthic foraminiferal assemblages and the shallow infaunal foraminifer Uvigerina peregrina δ13C record support the establishment of sustained high organic matter fluxes, and thus eutrophic conditions at the sea floor, during the late phase of the ORL (Younger Dryas to early Holocene periods). Since organic matter fluxes were lower (mesotrophic conditions) during the Bølling-Allerød period, they cannot be solely responsible for the ORL initiation. Geochemical, sedimentological and micropalaeontological proxies support a major weakening of the deep-water convection in the Gulf of Lion as the main driver for the development of poorly-ventilated conditions from intermediate depths (946 m) to the deep western Mediterranean basin that promoted the beginning of the ORL deposition. Nevertheless, a better ventilation at intermediate depths was established during the late ORL, while the deep basin remained poorly ventilated. We propose that our data reflect the arrival of a new better-ventilated intermediate water mass analogue to the current Levantine Intermediate Water (LIW) and/or a new intermediate water mass from the Gulf of Lion. The ultimate source of this water mass needs to be further explored but chronologies of the changes recorded here indicate that intermediate and deep ventilation phases were decoupled between the western and eastern Mediterranean basins during the deglaciation and early-middle Holocene.

Dissolved inorganic Radiocarbon content of the Western Coral sea: Implications for Intermediate and Deep Water Circulation

ABSTRACTThe South Pacific Ocean contributes to the global carbon cycle by exchanging CO2 between the atmosphere and intermediate to deep water masses. The path of the Antarctic Intermediate Water (AAIW) in the South Pacific gyre has been inferred from salinity, oxygen, and nutrient measurements, but radiocarbon (14C) measurements—a direct tracer of the carbon cycle—remain sparse. Here, we present the first radiocarbon profiles in the western Coral Sea and compare our measurements with South Pacific stations from GLODAPv2, a database of ocean hydrochemistry. Surface and subsurface waters in the Coral Sea cannot be attributed to a single source based on their Δ14C signatures, and we observe a penetration of bomb-produced 14C. AAIW in the western Coral Sea shows Δ14C values comparable to those in the South Pacific gyre, consistent with circulation of AAIW in the lower part of the southern equatorial current. The deep waters of the western Coral Sea have significantly higher 14C than the South Pacific at the same isopycnal, consistent with a northward intrusion of Circumpolar Deep Water from the Tasman Sea, along with a westward influx of deep waters from the Central Pacific. In accordance with silicate concentrations published previously, this shows the dual origin of deep waters in the Coral Sea.

Intermediate water circulation changes in the Florida Straits from a 35 ka record of Mg/Li-derived temperature and Cd/Ca-derived seawater cadmium

We generated a high-resolution ocean record of bottom water temperature and seawater Cd from 546 m in the Florida Straits. The record extends to ∼35,000 yr before present, including all of Marine Isotope Stages (MIS) 2 and part of MIS 3. Previously, seawater cadmium (Cdw) has been used as a proxy for the strength of the Atlantic Meridional Overturning Circulation (AMOC) during the last deglaciation. We now consider the temperature and oxygen isotopic composition of seawater reconstructed from the same core in order to provide a fuller picture of water mass properties and circulation at this location. During the Last Glacial Maximum (LGM), Cdw levels were generally low, and a novel Mg/Li-derived temperature record reveals persistently cold temperatures (∼3.6°C, two to three degrees colder than during the Holocene or Marine Isotope Stage 3). During the Younger Dryas and early Heinrich Stadial 1, Cdw was low and temperatures were elevated compared with periods of inferred stronger AMOC. By contrast, there is no indication of AMOC variability over Heinrich Stadials 2 and 3, consistent with previous studies that conclude the AMOC is less sensitive to freshwater forcing during glaciations than during the last deglaciation. While there is some inconsistency between proxies, Cdw increases over some MIS3 Dansgaard–Oeschger interstadials, providing qualified support for a strengthening in AMOC. This is similar to our previous finding that Cdw tracks AMOC variability during the deglaciation. This study highlights the distinct nature of water masses and circulation during the LGM, relative to the stadial (weak AMOC) periods of the deglaciation and Late MIS 3.

Sortable silt records of intermediate-depth circulation and sedimentation in the Southwest Labrador Sea since the Last Glacial Maximum

The Labrador Sea is a vital region for the Atlantic Meridional Overturning Circulation (AMOC), where overflow waters from the Nordic Seas mix with locally produced Labrador Sea Water (LSW), before exiting to the interior of the Atlantic Ocean. The dynamical sedimentary proxy of mean sortable silt size (SS¯) can give information on past changes in deep water circulation speed and the strength of AMOC. We have produced SS¯ records from two core sites at depths between 1500 and 2000 m on the continental slope east of Newfoundland, to reconstruct changes in intermediate depth water circulation speed, including Glacial North Atlantic Intermediate Water and Labrador Sea Water over the past 22,000 years. Increases in SS¯ appear to coincide with much of the deglaciation as well as the mid-late Holocene. End-member modeling suggests that ice-rafted debris (IRD) is an important factor in interpreting SS¯ during the deglaciation. We find that a robust increase in SS¯ is likely unrelated to IRD during the past 5 ka, and probably reflects increased flow at intermediate depths due to local production of LSW strengthening as Nordic Seas overflows weakened at this depth. Our results highlight both the complications of producing SS¯ records in IRD-rich, slope environments and the promise that this proxy nevertheless has for reconstructing dynamical changes in deep ocean currents.

Neogene Modifications of Circulation in the Northeastern African‐Southern Ocean Gateway

AbstractIn the African‐Southern Ocean gateway, several water masses originating in the Atlantic, Indian, and Southern oceans meet and mix. As a consequence, the gateway is crucial for the maintenance of the global thermohaline circulation. Newly acquired multichannel seismic reflection data collected across the southern Mozambique Ridge are used to reconstruct the impact of paleoceanographic modifications on the Neogene and Quaternary circulation in the northeastern African‐Southern Ocean gateway. The data show the occurrence of mid‐Miocene (~15 Ma) to early Pliocene (~5 Ma) contourite drifts and erosional features interpreted as evidence for the onset of current‐controlled sedimentation in the late Neogene resulting from mid‐Miocene cooling and closure of the Indonesian gateway. The Quaternary is characterized by a relocation of Antarctic Bottom Water inflow and the inception of two branches of the North Atlantic Deep Water circulation subsequent to the final closure of the Central American Seaway and the Northern Hemisphere Glaciation. Therefore, the two events triggered the onset of unhindered deep and bottom water circulation from the Atlantic into the Indian Ocean, whereas the Antarctic Intermediate Water circulation decreased due to the final closure of the Indonesian gateway. Our results show that tectonic and climatic events, which themselves may be linked, continuously modified the Cenozoic paleoceanic circulation in the African‐Southern Ocean gateway, and indicate that ocean gateways governing the global water mass exchange act as an excellent location to reconstruct such modifications based on the interpretation of contourite drifts and erosional features.

A multi-proxy analysis of Late Quaternary ocean and climate variability for the Maldives, Inner Sea

Abstract. As a natural sediment trap, the marine sediments of the sheltered central part of the Maldives Inner Sea represent an exceptional archive for paleoenvironmental and climate changes in the equatorial Indian Ocean. To evaluate the complex interplay between high-latitude and monsoonal climate variability, related dust fluxes, and regional oceanographic responses, we focused on Fe ∕ Al, Ti ∕ Al and Si ∕ Ca ratios as proxies for terrigenous sediment delivery and total organic carbon (TOC) and Br XRF counts as proxies for marine productivity. Benthic foraminiferal fauna distributions, grain size and stable δ18O and δ13C data were used for evaluating changes in the benthic ecosystem and changes in the intermediate water circulation, bottom water current velocity and oxygenation. Our multi-proxy data record reveals an enhanced dust supply during the glacial intervals, causing elevated Fe ∕ Al and Si ∕ Ca ratios, an overall coarsening of the sediment and an increasing amount of agglutinated benthic foraminifera. The enhanced dust fluxes can be attributed to higher dust availability in the Asian desert and loess areas and its transport by intensified winter monsoon winds during glacial conditions. These combined effects of wind-induced mixing of surface waters and dust fertilization during the cold phases resulted in an increased surface water productivity and related organic carbon fluxes. Thus, the development of highly diverse benthic foraminiferal faunas with certain detritus and suspension feeders was fostered. The difference in the δ13C signal between epifaunal and deep infaunal benthic foraminifera reveals intermediate water oxygen concentrations between approximately 40 and 100 µmol kg−1 during this time. The precessional fluctuation pattern of oxygen changes resembles that from the deep Arabian Sea, suggesting an expansion of the oxygen minimum zone (OMZ) from the Arabian Sea into the tropical Indian Ocean with a probable regional signal of strengthened winter-monsoon-induced organic matter fluxes and oxygen consumption further controlled by the varying inflow intensity of the Antarctic Intermediate Water (AAIW). In addition, the bottom water oxygenation pattern of the Maldives Inner Sea reveals a long phase of reduced ventilation during the last glacial period. This process is likely linked to the combined effects of generally enhanced oxygen consumption rates during high-productivity phases, reduced AAIW production and the restriction of upper bathyal environments in the Inner Sea during sea-level lowstands. Thus, our multi-proxy record reflects a close linkage between the Indian monsoon oscillation, intermediate water circulation, productivity and sea-level changes on orbital timescale.

Coherent Response of Antarctic Intermediate Water and Atlantic Meridional Overturning Circulation During the Last Deglaciation: Reconciling Contrasting Neodymium Isotope Reconstructions From the Tropical Atlantic

AbstractAntarctic Intermediate Water (AAIW) plays important roles in the global climate system and the global ocean nutrient and carbon cycles. However, it is unclear how AAIW responds to global climate changes. In particular, neodymium isotopic composition (εNd) reconstructions from different locations from the tropical Atlantic have led to a debate on the relationship between northward penetration of AAIW into the tropical Atlantic and the Atlantic meridional overturning circulation (AMOC) variability during the last deglaciation. We resolve this controversy by studying the transient oceanic evolution during the last deglaciation using a neodymium‐enabled ocean model. Our results suggest a coherent response of AAIW and AMOC: when AMOC weakens, the northward penetration and transport of AAIW decrease while its depth and thickness increase. Our study highlights that as part of the return flow of the North Atlantic Deep Water, the northward penetration of AAIW in the Atlantic is determined predominately by AMOC intensity. Moreover, the inconsistency among different tropical Atlantic εNd reconstructions is reconciled by considering their corresponding core locations and depths, which were influenced by different water masses in the past. The very radiogenic water from the bottom of the Gulf of Mexico and the Caribbean Sea, which was previously overlooked in the interpretations of deglacial εNd variability, can be transported to shallow layers during active AMOC and modulates εNd in the tropical Atlantic. Changes in the AAIW core depth must also be considered. Thus, interpretation of εNd reconstructions from the tropical Atlantic is more complicated than suggested in previous studies.

Pacific-Indian interocean circulation of the Antarctic Intermediate Water around South Australia

On the basis of the salinity distribution of isopycnal (σ 0=27.2 kg/m3) surface and in salinity minimum, the Antarctic Intermediate Water (AAIW) around South Australia can be classified into five types corresponding to five regions by using in situ CTD observations. Type 1 is the Tasman AAIW, which has consistent hydrographic properties in the South Coral Sea and the North Tasman Sea. Type 2 is the Southern Ocean (SO) AAIW, parallel to and extending from the Subantarctic Front with the freshest and coldest AAIW in the study area. Type 3 is a transition between Type 1 and Type 2. The AAIW transforms from fresh to saline with the latitude declining (equatorward). Type 4, the South Australia AAIW, has relatively uniform AAIW properties due to the semienclosed South Australia Basin. Type 5, the Southeast Indian AAIW, progressively becomes more saline through mixing with the subtropical Indian intermediate water from south to north. In addition to the above hydrographic analysis of AAIW, the newest trajectories of Argo (Array for real-time Geostrophic Oceanography) floats were used to constructed the intermediate (1 000 m water depth) current field, which show the major interocean circulation of AAIW in the study area. Finally, a refined schematic of intermediate circulation shows that several currents get together to complete the connection between the Pacific Ocean and the Indian Ocean. They include the South Equatorial Current and the East Australia Current in the Southwest Pacific Ocean, the Tasman Leakage and the Flinders Current in the South Australia Basin, and the extension of Flinders Current in the southeast Indian Ocean.

Holocene strengthening of the Oxygen Minimum Zone in the northwestern Arabian Sea linked to changes in intermediate water circulation or Indian monsoon intensity?

A high resolution 14,000year old record of benthic foraminifera from ODP Hole 723A was produced to understand changing intensity of the NW Arabian Sea Oxygen Minimum Zone (OMZ) and its links with intermediate water circulation and Indian Monsoon Intensity. During the latest Pleistocene, the OMZ was very weak marked by the abundance of oxic species. From early to mid Holocene period (up to 6500years BP), intense SW monsoon-related high surface productivity led to the development of an OMZ in the Arabian Sea; however, presence of oxic and suboxic species indicate that OMZ intensification was minimized by cyclic incursions of oxygen-rich intermediate waters from the South. The middle Holocene was a transitional period to intense OMZ conditions. The intensification of the OMZ could be due to replacement of oxygen-rich southern source Sub-Antarctic Mode and Antarctic Intermediate Waters (SAMW-AAIW), with oxygen poor Red Sea Water (RSW) from the north during the middle Holocene. As the intensity of the summer monsoon weakened during the late Holocene (~4200years BP onwards) the Arabian Sea OMZ intensified, suggesting total cut off from the oxygenated SAMW-AAIW. Spectral analysis of Information Function (H), benthic foraminifer Bolivina spp. and oxic species of benthic foraminifera indicate that the intensity of the OMZ and deep water conditions respond closely to solar cycles.