Composition Of Deep Waters Research Articles

Seawater Pb concentration and isotopic composition response to daily time scale dust storms in the Gulf of Aqaba, Red Sea

Lead (Pb) is a primary tracer of natural and anthropogenic processes in the marine environment, with atmospheric deposition being one of its main sources to open ocean surface waters. However, the impact of short-term (daily) dust storms on the oceanic water column is poorly constrained due to the typically low sampling temporal resolution in deep open ocean settings. The Gulf of Aqaba (GoA), northern Red Sea, is a deep oligotrophic water body surrounded by hyper-arid deserts with no major tributaries, and hence its surface waters receive limited terrigenous input, except from settling atmospheric dust particles. The GoA is highly accessible and therefore provides the opportunity to study the dynamics of seawater Pb chemistry across abrupt and short periods of enhanced atmospheric deposition.Here, we report a highly resolved time series of vertical profiles of dissolved Pb concentrations and isotopic compositions across daily-timescale dust storm events that occurred during 2018. GoA Pb concentrations range between 19 and 85 pmol kg−1, and the isotopic compositions of 206Pb/207Pb and 208Pb/206Pb are in the range of 1.163–1.190 and 2.062–2.093, respectively. These compositions reflect a mixture of multiple end members: seafloor sediments and open Red Sea waters, which modulate the long-term deep-water composition of GoA Pb, and settling aerosols, which shift the seawater composition towards anthropogenic values. The results show that dust storms impose strong perturbations of seawater compositions towards the anthropogenic aerosols end member, inducing increases of up to 54% in the Pb inventory in the upper water column. We propose a short-term dissolution-scavenging mechanism, whereby upon dust-seawater impact, the majority of the aerosol's soluble anthropogenic phases rapidly dissolve. The peak in the aerosol derived isotopic shift in the mixed layer is reached after 3–4 days. Simultaneously, Pb is re-adsorbed onto the sinking dust mineral particulates. Post dust storm Pb leaching and scavenging rates are modeled, yielding quantitative constraints on the connection between increasing dust loads and seawater Pb concentrations and isotopic compositions. Our findings demonstrate the short-term variable Pb response to dust storms and should be applied when interpreting open ocean surface water Pb patterns.

The chromium isotope composition of reducing and oxic marine sediments

The chromium (Cr) isotope composition of marine sediments has the potential to provide new insights into the evolution of Earth-surface redox conditions. There are significant but poorly constrained isotope fractionations associated with oxidative subaerial weathering and riverine transport, the major source of seawater Cr, and with partial Cr reduction during burial in marine sediments, the major sink for seawater Cr. A more comprehensive understanding of these processes is needed to establish global Cr isotope mass balance and to gauge the utility of Cr isotopes as a paleoredox proxy. For these purposes, we investigated the Cr isotope composition of reducing sediments from the upwelling zone of the Peru Margin and the deep Cariaco Basin. Chromium is present in marine sediments in both detrital and authigenic phases, and to estimate the isotopic composition of the authigenic fraction, we measured δ53Cr on a weakly acid-leached fraction in addition to the bulk sediment. In an effort to examine potential variability in the Cr isotope composition of the detrital fraction, we also measured δ53Cr on a variety of oxic marine sediments that contain minimal authigenic Cr. The average δ53Cr value of the oxic sediments examined here is −0.05±0.10‰ (2σ, n=25), which is within the range of δ53Cr values characteristic of the bulk silicate Earth. This implies that uncertainty in estimates of authigenic δ53Cr values based on bulk sediment analyses is mainly linked to estimation of the ratio of Cr in detrital versus authigenic phases, rather than to the Cr-isotopic composition of the detrital pool. Leaches of Cariaco Basin sediments have an average δ53Cr value of +0.38±0.10‰ (2σ, n=7), which shows no dependency on sample location within the basin and is close to that of Atlantic deepwater Cr (∼+0.5‰). This suggests that authigenic Cr in anoxic sediments may reliably reflect the first-order Cr isotope composition of deepwaters. For Peru Margin samples, the average δ53Cr values of bulk sediments (+0.59±0.06‰; 2σ) and leach fractions (+0.61±0.06‰; 2σ) are also comparable with those of Cariaco Basin samples and modern deepwater values. Finally, we found that the δ53Cr of Peru Margin samples correlates with δ15N on glacial–interglacial timescales, which we attribute to secular variation in basinal or global-ocean redox conditions. Thus, the δ53Cr stratigraphic record of Peru Margin sediments indicates that Cr isotopes may be suited to tracking geologically short-term changes in ocean oxygenation.

Northern source for Deglacial and Holocene deepwater composition changes in the Eastern North Atlantic Basin

Over the last decades extensive research has been carried out on changes in the Atlantic Meridional Overturning Circulation and its role during the last deglacial period. At present, only a few high-resolution data sets from the deep eastern North Atlantic exist for this period. It therefore remains uncertain whether deepwater changes in the Eastern North Atlantic Basin were governed by alternating contributions of northern and southern deepwater or whether the Eastern North Atlantic Basin reflects changes in the initial composition and source of North Atlantic deepwater. Furthermore, it is still debated whether such changes are triggered by Northern or Southern Hemisphere climatic changes.In this centennial to decadal scale resolution study we investigate deepwater composition changes in the Eastern North Atlantic Basin over the last 15 ka BP. We used sediment cores GEOFAR KF16 and MD08-3180 (37.984° N; 31.118° W, wd 3050 m/37.999° N; 31.134° W, wd 3064 m), obtained from a small basin at the eastern flank of the Mid Atlantic Ridge south of the Azores Islands. Under modern conditions the coring site is situated at the boundary between southern Lower Deep Water and northern Eastern North Atlantic Deep Water consisting of Iceland Scotland Overflow Water and Labrador Sea Water. Distinct differences between the three water masses in terms of ventilation state, temperature and salinity signatures are ideal for tracking changes in the deglacial North Atlantic deepwater distribution using paired benthic foraminiferal stable isotopes (δ13C, δ18O) and Mg/Ca bottom water temperature reconstructions.The results show a close coupling of low bottom water temperature (1.5 °C) and δ13C(0–0.5‰) values during cold Heinrich event 1, the Younger Dryas and the Preboreal, that were coeval with a major δ18O depletion of deepwater. The strong similarities between subtropical Eastern North Atlantic Deep Water and subpolar North Atlantic surface and deepwater changes indicate that deglacial changes in Eastern North Atlantic Deep Water distribution were triggered in the North Atlantic. Consequently, this would imply that changes in the North Atlantic also contributed to deglacial changes in Antarctic bottom water composition. During the early Holocene stepwise increasing δ13C values suggest increasing Eastern North Atlantic Deep Water production and/or increasing Eastern North Atlantic Deep Water ventilation with minor but distinct interruptions (at 10.8, 10.6, 9.1, 8.4, 8.1 and 7.2 ka BP), which were most probably also triggered in the subpolar North Atlantic.

Radiogenic Nd isotope labeling of the northern NE Atlantic during MIS 2

Paleoceanographic reconstructions rely on chemical proxies which are controlled by physical, chemical, and biological marine parameters. The accurate interpretation of proxy records relies on the integrity of proxy-environmental relationships through time, and under changing conditions. In this study we closely examine paleo controls on authigenic Nd isotope records from five cores in the northern NE Atlantic, approximating a depth-transect, allowing spatial and temporal relationships to be reconstructed. We compare our Nd isotope records with other paleocirculation proxies, and consider the sedimentalogical controls on Nd isotope signals, by comparing ice-rafted detritus lithology and counts, detrital sediment chemistry and redox sensitive element concentrations measured on foraminifera authigenic coatings. With this suite of geochemical and sedimentalogical data we show that Nd isotope records in the northern NE Atlantic were labeled by radiogenic sediments, however this modification did not occur in the pore-waters of each core, but instead likely reflects changes in the Nd isotopic composition of deep-waters caused by the input of ice-rafted sediment during Heinrich events and the last glacial maximum. This study has implications for understanding how localized changes in the Nd isotope signal can set a watermass end-member composition, decoupling chemical proxy-circulation relationships locally, but providing a signal which can be potentially traced along the deep-water flowpath. Such scenarios must be considered in future interpretations of glacial Nd isotope records taken from within the ice-rafted detritus belt and downstream along watermass flowpaths.

Water mass analysis for the U.S. GEOTRACES (GA03) North Atlantic sections

We present the distributions of hydrographic properties (potential temperature, salinity, dissolved oxygen, and micromolar level inorganic macronutrients) along two sections occupied in the subtropical North Atlantic as part of the first U.S. GEOTRACES (GA03) survey during 2010 and 2011. The purpose of this work is to place subsequent papers in this special issue in a general context and to provide a framework in which the observed distributions of Trace Elements and Isotopes can be interpreted. Using these hydrographic properties we use a modified Optimum Multiparameter water mass analysis method to diagnose the relative contributions of various water types along the sections and rationalize their distributions. The water mass compositions appear largely consistent with what is understood from previous studies about the large scale circulation and ventilation of the North Atlantic, with perhaps one exception. We found that the North Atlantic Deep water both east and west of the Mid Atlantic Ridge is more strongly influenced by Iceland Scotland Overflow Water relative to Denmark Straits Overflow Water (about 3:1) than inferred from other tracer studies (typically 2:1). It remains unclear whether this is an artifact of our calculation or a real change in deep water composition in the decades between the determinations.

Deep water composition and water mass mixing in the Paleogene North Atlantic; Results from the Newfoundland ridges

Deep water composition and water mass mixing in the Paleogene North Atlantic; Results from the Newfoundland ridges

The Eocene-Oligocene turnover of Deep-Water Agglutinated Foraminifera at ODPSite 647, Southern Labrador Sea (North Atlantic)

We present a new record of deep-water agglutinated foraminifera (DWAF) across the Eocene-Oligocene Transition (EOT) in the southern Labrador Sea (ODP Site 647). We studied 82 samples from Cores 647A-37R to -27R, and recovered over 100 species and generic groups. The EOT represents an interval of rapid climatic change connected with global cooling, Antarctic glaciation, a substantial decrease in atmospheric CO2, and concurrent changes in the composition of deep waters in the world ocean. Our high-resolution quantitative study of the DWAF faunal succession in abyssal Hole 647A confirms earlier findings that the EOT was an interval of significant faunal turnover. The faunal succession in Hole 647A is subdivided into two assemblages based on the stratigraphic ranges of characteristic benthic foraminiferal species: late Eocene Spiroplectammina trinitatensis-Reticulophragmium amplectens Zone and early Oligocene Ammodiscus latus-Turrilina alsatica Zone. The boundary between these zones, i.e., the Eocene/Oligocene (E/O) boundary, is characterized by the disappearance of 11 DWAF taxa, most of them organically-cemented taxa. The boundary interval was also characterized by a striking sharp decrease in DWAF abundance and diversity. Organically-cemented DWAF taxa increased in abundance in the early Oligocene, but their diversity and abundance never recovered to Eocene values. These data suggest a deepening of the calcite compensation depth and associated changes, such as more vigorous ocean circulation in coincidence with the E/O boundary interval. The analysis of DWAF morphogroups reveals an acme in robust suspension-feeding tubular forms previous to the extinction, suggesting increased bottom water activity. The E/O boundary interval is characterized by an increase in DWAF infaunal taxa and Spiroplectammina species, probably related to increased productivity, as already suggested by the analysis of benthic elongate-cylindrical foraminifera at the same locality. The faunal turnover across the EOT at Hole 647A suggests more vigorous deep-ocean circulation in the latest Eocene and across the E/O boundary interval in the Labrador Sea. It seems reasonable to link the disappearance of DWAF at the E/O boundary at Site 647 to more than one mechanism, including inferred higher productivity, possible competition from calcareous benthic foraminifera, changes in the CO2 content of the atmosphere and ocean, the deepening of the calcite compensation depth, and concurrent changes in taphonomic conditions caused by changes in the water masses in the North Atlantic.

Influence of deep gas-steam fluids on the composition of reservoir waters of oil and gas fields

In this work the conditions of the formation of low-mineral deep hydrocarbonate sodium waters of oil-and-gas fields are considered. Analysis of the boron-bromine ratio of reservoir and condensed waters in several fields of Western Siberia shows that the influx of endogenous high-temperature gas-steam fluids into zones containing sedimentogenic brines is a decisive factor in the formation of the chemical composition of deep waters that are low in minerals.

Pb isotope variations in hydrogenetic Fe–Mn crusts from the Izu–Bonin fore-arc

Fe–Mn crusts were recovered from the western escarpment of the Bonin Ridge in the Izu–Bonin fore-arc region (dive site #824: 28.612°N, 141.803°E) at water depths of c. 2900 m using the Shinkai 6500 submersible during cruise YK 04–05. Major and trace element data and XRD mineralogy indicate that the crusts are hydrogenetic in origin. We present profiles of variations in Pb isotope composition measured in-situ by laser ablation MC-ICP-MS across two of the crusts. The isotopic variations are systematic and can be matched up between the two crusts, indicating similar growth rates. The crust Pb isotope composition rules out any local source for Pb from within the Izu–Bonin–Mariana arc system, either from hydrothermal activity or through leaching of volcanic detritus. Input of a globally well-mixed volcanic Pb component, either from aerosols or as an absorbed component on aeolian dust, has been proposed as a mechanism to explain the Pb isotope composition of Central Pacific deep water. However, the Izu–Bonin crusts are displaced to lower 206Pb/ 204Pb and higher 208Pb/ 204Pb, which requires an additional Pb source. One possibility is that as water is advected from the south, outboard of the Luzon–Ryukyu–Honshu arc system, it is progressively polluted by Pb derived by weathering and erosion of these young island arc volcanic systems. Using a constant Co-flux model, growth rates are estimated at ∼ 7–13 mm/Ma, which would suggest that these crusts provide a record of changes in the composition of deep water in the Izu–Bonin fore-arc region of the western Pacific Ocean over the last 4–8 Ma. Over this interval, the main feature has been a progressive decrease in 207Pb/ 206Pb (0.843 to 0.839) and 208Pb/ 206Pb (2.088 to 2.080) with time. The interior parts have compositions similar to those of crusts from the Izu–Bonin fore-arc, while the rims have compositions similar to crusts from the central Western Pacific.

Paleogene deepwater mass composition of the tropical Pacific and implications for thermohaline circulation in a greenhouse world

The Pacific Ocean likely exerted the greatest marine influence on climate during the greenhouse interval of the Paleogene. The first step toward assessing the role of Pacific deepwater circulation in global heat transport is a comprehensive reconstruction of deepwater mass composition and circulation patterns. We present new deepwater mass Nd isotope data from ODP Leg 199 Sites 1215, 1217, 1219, and 1221 to augment the existing coverage provided by ferromanganese crust and fossil fish debris data. The new data corroborate the notion that the Pacific Ocean operated in a bimodal pattern with deepwater sources in both the Southern Ocean and North Pacific for much of the early to middle Paleogene. In addition, the data indicate that there was little to no influence of Atlantic waters on the deepwater composition of the tropical Pacific. The available deepwater Nd isotope data provide refined constraints on the geographic and depth range of influence of both a Southern Ocean and North Pacific deepwater mass. From ∼65 to ∼45 Ma, the deep tropical Pacific was the mixing location of deep waters sourced from the Southern Ocean and the North Pacific. The Southern Ocean deepwater mass extended from ∼1800 m to abyssal depths. Until 53.6 Ma it circulated northward in the western Pacific at least as far as the paleolocation of Fe‐Mn Crust CLD01 (∼5°N paleolatitude) and in the eastern portion of the Pacific as far as Site 1215 (∼10°N paleolatitude). A North Pacific deepwater mass extended from at least ∼2400 m to 2900 m water depth from ∼65 to 54 Ma, south to Shatsky Rise (paleolatitude ∼10–15°N), and possibly as far as Site 1215, 700 km to the south of Shatsky Rise Sites 1209 and 1211.

Indications for control of the Iceland plume on the Eocene–Oligocene “greenhouse–icehouse” climate transition

The Eocene/Oligocene boundary, at about 33.5 Myr ago, marks the transition from ‘greenhouse-’ to ‘icehouse-world’, accompanied by a sudden cooling of ocean bottom-water. We show that this global event is simultaneous with a deep rooted mantle process: an abrupt suppression of the Iceland plume triggered rapid deepening of the Greenland–Scotland Ridge (GSR) — the sill moderating deep circulation between the Nordic seas and North Atlantic. Striking coincidence of several sets of events reflects the abrupt suppression of the Iceland plume and a rapid removal of its influence on the nearby Reykjanes Ridge (RR): 1) A sudden segmentation of the paleo-RR seen on seafloor magnetic anomalies, 2) a drop in spreading rate of the North Atlantic, 3) a transition from thick to normal oceanic crust, and 4) a rapid deepening and accelerated subsidence of the GSR, inferred from the sedimentary record of DSDP site 336. The plume suppression and the concomitant GSR deepening coincide with the initiation of North Atlantic Deep Water (NADW) at the Eocene/Oligocene (E/O) transition, attested by onset of drift sedimentation in the Faroe–Shetland Channel (FSC), the deepest spill-point on the GSR, and in the North Atlantic, the Feni Drift. These processes have influenced global deepwater composition and temperature as indicated by the striking correlation with the jump in global δ 18O (> 1‰) measured on benthic foraminifers that reflects the E/O global cooling, and with enrichment of unradiogenic Nd isotopes in the southeastern Atlantic and Southern Ocean. The initiation of Atlantic thermohaline circulation at that time is inferred from the abrupt split between planktonic and benthic δ 18O, indicating the building of ocean-water stratification. This scenario is further corroborated by a reversal in benthic δ 18O at the late Oligocene, coincident with the renewal of vigorous Iceland plume some 25 Myr ago, causing a considerable retardation in NADW fluxes. The plume renewal is inferred from the emergence of the Iceland plateau, the transition to oblique-unsegmented RR axis, the cessation in deepening of the GSR, and rapid increase in spreading rate of the North Atlantic. These events coincide with decreasing difference in planktonic–benthic in global δ 18O by the late Oligocene. All these inferences suggest the role of the NADW sourced at the Nordic seas to form background cooler conditions in the long time scale since the early Oligocene, or to form permanent conditions of invigorated thermohaline circulation that forces CO 2 trap in the oceans.

Calcite dust and the atmospheric supply of Nd to the Japan Sea

We report neodymium concentrations and isotope ratios ( ε Nd) measured in the carbonate and silicate fractions of Chinese loess. The goal of the study is to investigate if the atmospheric supply of dissolved calcite dust to the oceans influences the cycling of Nd in seawater. We primarily focus on the Japan Sea, because it receives large quantities of dust from eastern Asia, and it has a known Nd isotope composition and a simple hydrologic budget. A steady-state atmospheric transport model is used to calculate the flux of soluble Nd delivered to the Japan Sea surface by the wet deposition of dissolved calcite dust. This information is then incorporated in a steady-state two-box model in order to evaluate controls on dissolved ε Nd values as well as to calculate the residence time of Nd in seawater. On average, calcite in fresh Chinese loess contains ∼150 nmol of Nd g − 1 with an ε Nd of − 8.6. The atmospheric input of dissolved calcite to the Japan Sea surface is 7.4 × 10 12 g yr − 1 , which corresponds to a dissolved Nd flux of 1.1 × 10 18 pmol yr − 1 . This flux represents the largest input of Nd to the basin, and it is required to produce the ε Nd composition of Japan Sea waters. Approximately 65% of the Nd flowing from the Japan Sea surface into the western North Pacific originates directly from the input of dissolved calcite dust. Downwelling surface water and the remineralization of settling particles containing Nd scavenged from surface water contribute ∼77% of the Nd in the Japan Sea deep, which indicates that ∼50% of the deep Nd originates from calcite dust. The ε Nd composition of deep water is balanced by a benthic flux, but the input is a factor of ∼10 lower than the calcite dust flux and accounts for only ∼23% of the deep Nd. Surface and deep waters in the Japan Sea have Nd residence times, with respect to particle scavenging, of ∼14 and 530 yr, respectively. These findings reveal that the atmospheric supply of dissolved calcite dust to the Japan Sea is a more important component of the marine Nd cycle in this basin than previously recognized.

Preparation and assessment of a candidate reference sample of Lake Baikal deep water

The possibility of the creation of a multi-element reference sample of Lake Baikal deep-water composition is justified. This is a new type of reference sample composed of natural water with a wide range of macro- and microelements. This candidate reference sample has a matrix composition consisting of hydrocarbonate and calcium water, a composition that is typical of many rivers and lakes of the world, as well as rain water. The creation of a candidate reference sample of Lake Baikal water is possible due to the stable water composition at a depth of 500 m, and to the use of water sampling technology which results in the preservation of the initial composition of water and its absolute sterility. Trial batches of Baikal water collected annually and stored in special polyethylenetereftalate bottles for a period of 9 years remained stable and homogenous for most elements. Preliminary data for a range of elements and compounds are presented.

Early diagenetic processes during Mn-carbonate formation: evidence from the isotopic composition of authigenic Ca-rhodochrosites of the Baltic Sea

The formation of authigenic Ca-rich rhodochrosite (ACR) in sapropelic sediments of the Gotland Basin, Baltic Sea, is governed by deepwater renewal processes whereby saline water from the North Atlantic flushes the brackish anoxic Baltic Deeps. The carbon and oxygen isotopic compositions of these Mn-carbonates suggest that ACR formation takes place just below the sediment surface and that dissolved compounds from the deepwater column, such as water and bicarbonate molecules, were incorporated in ACR during authigenesis. Porewaters near the sediment surface display δ18O values of −5.4‰ (VSMOW) and are generally depleted in 18O, compared to the oxygen isotopic composition of water in equilibrium with Mn-carbonate solid solutions (ACR δ18O values are −4.6‰). This suggests that early burial diagenetic processes significantly modify the initial isotopic composition of water during Mn-carbonate formation. The reduction of sulfate having δ18O values of +8.4‰ accounts for a permanent enrichment of porewater 18O and observed δ18O values at depth equal to −4.6‰. However, this process does not explain the observed disequilibrium in the oxygen isotopic composition between water and ACR close to the sediment surface where Mn-carbonate formation takes place. Based on isotopic mass balance calculations, we suggest that MnO2 with δ18O values of +8.9‰ released oxygen enriched in 18O into the anoxic porewaters close below the sediment surface. This process should occur after oxygenation events during deepwater renewal when MnO2 accumulates at the surface of anoxic sediments. Manganese carbonates formed in these waters display δ18O values of ∼1.0‰ heavier than values expected solely from the initial deepwater composition. This quantitatively explains the discrepancy between paleosalinities calculated from ACR δ18O based on Mn-carbonate/water isotopic equilibrium fractionation and direct observations for the same period. Our results emphasize the important role of microbial MnO2 reduction during rhodochrosite authigenesis and suggest that Mn(II) activity, rather than alkalinity, is the limiting component for sedimentary Mn-carbonate formation.

Secular variation of Nd and Pb isotopes in ferromanganese crusts from the Atlantic, Indian and Pacific Oceans

Two ferromanganese crusts from the Indian Ocean and one from the Atlantic Ocean have been analysed for 10Be/ 9Be, 143Nd/ 144Nd and 208,207,206Pb/ 204Pb ratios as a function of depth beneath their growth surfaces. 10Be/ 9Be ratios provide growth rate estimates for these crusts between 1.55 and 2.82 mm Ma −1 and further suggest that 87Sr/ 86Sr in crusts do not in any case examined so far provide reliable estimates for growth rates. A crust ALV-539 from 35°N in the western N. Atlantic has ϵ Nd and Pb-isotope variations indistinguishable from crust BM-1969.05 from 39°N in the N. Atlantic [K.W. Burton, H.-F. Ling, R.K. O'Nions, Closure of the central American isthmus and its impact on North Atlantic deepwater circulation, Nature (London) 386 (1997) 382–385] when 10Be/ 10Be ratios are used to estimate growth rates. Both crusts provide evidence for a marked change in deepwater composition in the western N. Atlantic with a reduction in ϵ Nd and an increase in 206Pb/ 204Pb from ∼8 Ma ago towards the present day. The two crusts from the Indian Ocean show comparatively small variations in ϵ Nd between −8.0 and −7.0 over the last 20 Ma and do not show the large shift in ϵ Nd seen in the Atlantic crusts. Comparison of ϵ Nd in the crusts analysed here with those published previously [H.-F. Ling, K.W. Burton, R.K. O'Nions, B.S. Kamber, F. von Blanckenburg, A.J. Gibb, J.R. Hein, Evolution of Nd and Pb isotopes in central Pacific seawater from ferromanganese crusts, Earth Planet. Sci. Lett. 146 (1997) 1–12; K.W. Burton, H-F. Ling, R.K. O'Nions, Closure of the central American isthmus and its impact on North Atlantic deepwater circulation, Nature (London) 386 (1997) 382–385] shows that provinciality in the present-day ϵ Nd structure of the Pacific, Atlantic and Indian Oceans has been maintained over ∼20 Ma or more despite the palaeogeographic changes that have occurred within this period. These include the closure of the Panama gateway and the uplift of the Himalayas. Superimposed on this broad inter-ocean structure are changes in ϵ Nd of the western N. Atlantic which may relate to the Panama gateway closure and shifts in the ϵ Nd of equatorial Pacific deepwater from 3–5 Ma ago. The absence of any such structure in ϵ Nd of the southwest and central Indian Ocean suggests that Himalayan erosion products such as preserved in the Bengal Fan sediments have not contributed significantly to Indian Ocean deepwater over the last 20 Ma. There is no straightforward relationship between records of 87Sr/ 86Sr in the global ocean and ϵ Nd in ocean deepwater as would be expected if inputs of radiogenic Sr and unradiogenic Nd were coupled.

Long-term variation in seawater composition at the base of the thermocline

Measurements of the chemical composition of seawater over a time scale of years at a fixed point in the ocean below the main thermocline have not been previously reported because of navigational limitations, ship and hydrowire instability, and, to a large extent, cost. The deep-water pipeline at the Natural Energy Laboratory of Hawaii at Keahole Point (Fig. 1a) provides a unique opportunity to address the question of time-varying water composition at a fixed site in the ocean below the mixed layer. In this study we examine the composition of sub-thermocline seawater through weekly sampling (surface-water data are also available, but are not discussed here). The mean deep-water composition obtained over about five years is representative of regional mean-water composition, and significant variation in water composition is observed on time scales exceeding one year. This unexpected long-term variation probably represents cyclic displacement of deep water masses in response to variation in the regional ocean circulation.