Present day Nd isotopic composition of seawater and sediment leaches from the Pacific sector of the Southern Ocean

ABSTRACT FINAL ID: PP11B-1785 The Southern Ocean plays a key role in the climate evolution of Earth’s history, nevertheless its biggest region, the Pacific sector, has been poorly investigated in comparison to other regions in many paleoceanographic aspects. Some of them will try to be explained by the SOPATRA Project (SOuth PAcific TRAnsects). One of those aspects is radiogenic isotope compositions. Hereby we present the first εNd data extracted from Fe-Mn hydroxide coatings of bulk sediments from the Pacific sector of the Southern Ocean. These surface sediments results, scattered along a longitudinal transect of about 10,000 miles, collected from cruise SO213 from middle Chile to New Zealand (between 36°S and 45°S), represent a reliable overview of the present day Nd isotopic composition of the ambient water masses in this region, and will provide useful information for the interpretation of later downcore studies of this water mass tracer. Multiple analysis were carried out to confirm the reliability of the data as well as the validity of the applied leaching method: 1) Measurements of the present day radiogenic Nd bottom water signatures were compared to the Nd isotope compositions of the leachates in order to verify their seawater origin. 2) The leachates’ 87Sr/86Sr isotope ratios were monitored to confirm the holocenic origin of the samples. 3) Radiogenic Nd and Sr compositions of the detrital fraction of the sediment were measured after total dissolution of the same set of samples in order to exclude detrital contamination of leachates. 4) Two different leaching procedures were tested. Preliminary results show εNd values between -4 and -6, corresponding the lowest values to central south Pacific, which could represent the imprint of the circumpolar deep water moving northward. These findings are in agreement with other authors who extracted bottom water Nd isotope compositions from manganese nodules

Deep-water circulation and detrital provenance in the South Pacific, from the present day until 240 000 years ago : evidence from Nd, Sr and Pb isotopes and Rare Earth Elements

Present and past climate of the Earth has strongly depended on oceanic circulation and marine biological productivity. The formation of deep and bottom waters in high latitudes as a consequence of density changes and their pathways through the global ocean, the so called thermohaline circulation (THC) have been of primary importance for the redistribution of heat, for the Earth’s albedo through control on the sea ice distribution, and it serves as a reservoir of greenhouse gases such as CO2. The understanding of the mechanisms that have driven the THC in the past is crucial to reliably predict future climatic variations. For example, large amounts of CO2 were stored in the deep ocean during glacial periods, which depended on the structure of the water column but also on the availability of nutrients for primary producers in the surface ocean. Besides macronutrients, phytoplankton depends also on dust input to the ocean, which releases iron, one of the most important micronutrients, in particular in High Nutrient Low Chlorophyll areas. These issues have been poorly studied in the South Pacific, despite its importance for these processes. It represents a key area for the interchange of deep waters from all ocean basin because it is the main entrance and exit of deep waters to the largest of all oceans, the Pacific, which is also one of the principal CO2 reservoirs on Earth. For the study of present and past deep-water circulation regimes and the provenance of the dust input in this region, Rare Earth Element (REE) distributions and radiogenic isotopes of neodymium (Nd), lead (Pb) and strontium (Sr) have been analyzed in water and sediment samples obtained from a meridional transect of the South Pacific, spanning all the way from South America to New Zealand. Radiogenic isotopes have been proved to be very reliable traces for studying different surface earth processes, such as, in the case of Nd and Pb, the advection of water masses. These are labeled with characteristic isotope compositions through weathering of the lithologies of the surrounding continents in their formation regions, allowing to track the pathway of a certain water mass in the present day water column (not in the case of Pb due to anthropogenic inputs), as well as their presence and mixing in the past at a particular location given that these signatures are recorded by the sediments. Nd, Sr and Pb isotopes also allow identifying the provenance of lithogenic particles that arrive the bottom of the ocean brought by currents or wind due to the specific signature that different rocks carry as a consequence of their type and age. The concentrations of the REE including Nd in seawater also allow to distinguish water masses as well as vertical processes such as scavenging as their relative distributions vary coherently in the water column due to different affinities to particles. Chapter 4 of this study presents the first seawater REE concentrations and Nd isotope compositions in intermediate and deep waters of the South Pacific. The results show that Nd isotopes faithfully trace the different water masses displaying more negative Nd isotope compositions for those water masses originating in the Southern Ocean, such as Lower Circumpolar Deep Water (LCDW) and Antarctic Intermediate Water (AAIW), with εNd around -8.3; and more positive signatures for North Pacific Deep Water (NPDW)(εNd = -5.9), which exits the South Pacific in the east, close to South America. Nd isotope compositions also allowed identifying a remnant of North Atlantic Deep Water (NADW) entering the western South Pacific as part of LCDW. Dissolved REE concentrations indicate that NPDW were affected by scavenging processes underneath the high productivity area of the equatorial eastern Pacific that lowered the concentrations of the more particle reactive light REE (LREE) of this water mass before reaching the South Pacific. At the same time LREE are also released from oxides in the sediments of the Southeast Pacific Basin. Chapter 5 compares bottom seawater (chapter 4) and surface sediment Nd isotope compositions to identify the most reliable technique to obtain seawater-derived Nd isotopes from the sediment for the study of past circulation changes recorded in the sediment. Four different archives were tested in order to obtain the authigenic seawater Nd isotope signal that precipitates from seawater into the sediment in the form of early-diagenetic Fe-Mn oxide coatings: ‘decarbonated’ and ‘non-decarbonated’ bulk sediment leachates as well as ‘uncleaned’ planktonic foraminifera, which were also compared to the compositions of fossil fish teeth. None of them registered exactly the same Nd isotope compositions as those of the present day bottom waters due to the low sedimentation rates in the South Pacific that result in very old surface sediments (up to 24 kiloyears before present), which therefore integrate Nd isotope compositions from different circulation states of the ocean. However, independent evidence (REE patterns and Al/Ca ratios measured on unclean foraminifera), clearly indicates that the Nd isotope compositions of unclean foraminifera, fish teeth and ‘non-decarbonated’ leachates originated from bottom seawater, allowing the use of these methods to study past changes of deep water circulation. This study also suggests that contributions of the continental margins of the South Pacific to the Nd isotope composition of seawater, the so called ‘boundary exchange’ was small. The sources of fine lithogenic particles that arrive the South Pacific are studied in this chapter by combining Nd and Sr isotopes from the detrital fraction of the sediment. The results show that the lithogenic material found in the western and central Pacific originates from Southeast Australia and South New Zealand and was transported by the dominant Westerlies. The influence of these sources is also dominant in the eastern South Pacific, although in this region the proportions of detrital material from the Andes increase and contributions from Antarctica can also not be excluded. The last chapter of this thesis presents a reconstruction of the deep-water circulation and detrital provenances in the central South Pacific of the last 240 kyr based on Nd, Pb and Sr isotopes. The results show small but significant glacial-interglacial variations in the Nd and Pb isotope composition of the circumpolar deep water (CDW), which has been the dominating water mass in this region, caused by a decrease in the contribution of NADW during glacial stages. A deepening of the flow and advective incorporation of NPDW during glacial periods is not supported by the results of this study due to the location of the cores, which have remained within CDW. The combined detrital Nd-Sr isotope compositions indicate a dominance of Southeastern Australia and South New Zealand derived material, similar to the surface sediments over the past 240,000 years. Nevertheless, a small shift towards the Antarctic end-member during glacial periods is identified, which may indicate a stronger influence of Antarctic-derived material during cold periods, most probably transported northwards as suspended load of oceanic currents.

Upper ocean vertical supply: A neglected primary factor controlling the distribution of neodymium concentrations of open ocean surface waters?

Neodymium (Nd) isotopes are an important geochemical tool to trace the present and past water mass mixing as well as continental inputs. The distribution of Nd concentrations in open ocean surface waters (0–100 m) is generally assumed to be controlled by lateral mixing of Nd from coastal surface currents and by removal through reversible particle scavenging. However, using 228Ra activity as an indicator of coastal water mass influence, surface water Nd concentration data available on key oceanic transects as a whole do not support the above scenario. From a global compilation of available data, we find that more stratified regions are generally associated with low surface Nd concentrations. This implies that upper ocean vertical supply may be an as yet neglected primary factor in determining the basin-scale variations of surface water Nd concentrations. Similar to the mechanism of nutrients supply, it is likely that stratification inhibits vertical supply of Nd from the subsurface thermocline waters and thus the magnitude of Nd flux to the surface layer. Consistently, the estimated required input flux of Nd to the surface layer to maintain the observed concentrations could be nearly two orders of magnitudes larger than riverine/dust flux, and also larger than the model-based estimation on shelf-derived coastal flux. In addition, preliminary results from modeling experiments reveal that the input from shallow boundary sources, riverine input, and release from dust are actually not the primary factors controlling Nd concentrations most notably in the Pacific and Southern Ocean surface waters

Nd and Sr isotope compositions of different phases of surface sediments in the South Pacific: Extraction of seawater signatures, boundary exchange, and detrital/dust provenance

The radiogenic isotope composition of neodymium (Nd) and strontium (Sr) are useful tools to investigate present and past oceanic circulation or input of terrigenous material. We present Nd and Sr isotope compositions extracted from different sedimentary phases, including early diagenetic Fe-Mn coatings, ‘‘unclean’’ foraminiferal shells, fossil fish teeth, and detritus of marine surface sediments (core-tops) covering the entire midlatitude South Pacific. Comparison of detrital Nd isotope compositions to deep water values from the same locations suggests that "boundary exchange" has little influence on the Nd isotope composition of western South Pacific seawater. Concentrations of Rare Earth Elements (REE) and Al/Ca ratios of "unclean" planktonic foraminifera suggest that this phase is a reliable recorder of seawater Nd isotope composition. The signatures obtained from fish teeth and "nondecarbonated" leachates of bulk sediment Fe-Mn oxyhydroxide coatings also agree with "unclean" foraminifera. Direct comparison of Nd isotope compositions extracted using these methods with seawater Nd isotope compositions is complicated by the low accumulation rates yielding radiocarbon ages of up to 24 kyr, thus mixing the signal of different ocean circulation modes. This suggests that different past seawater Nd isotope compositions have been integrated in authigenic sediments from regions with low sedimentation rates. Combined detrital Nd and Sr isotope signatures indicate a dominant role of the Westerly winds transporting lithogenic material from South New Zealand and Southeastern Australia to the open South Pacific. The proportion of this material decreases toward the east, where supply from the Andes increases and contributions from Antarctica cannot be ruled out

The influence of basaltic islands on the oceanic REE distribution: A case study from the tropical South Pacific

The Rare Earth Elements (REEs) have been widely used to investigate marine biogeochemical processes as well as the sources and mixing of water masses. However, there are still important uncertainties about the global aqueous REE cycle with respect to the contributions of highly reactive basaltic minerals originating from volcanic islands and the role of Submarine Groundwater Discharge (SGD). Here we present dissolved REE concentrations obtained from waters at the island-ocean interface (including SGD, river, lagoon and coastal waters) from the island of Tahiti and from three detailed open ocean profiles on the Manihiki Plateau (including neodymium (Nd) isotope compositions), which are located in ocean currents downstream of Tahiti. Tahitian fresh waters have highly variable REE concentrations that likely result from variable water–rock interaction and removal by secondary minerals. In contrast to studies on other islands, the SGD samples do not exhibit elevated REE concentrations but have distinctive REE distributions and Y/Ho ratios. The basaltic Tahitian rocks impart a REE pattern to the waters characterized by a middle REE enrichment, with a peak at europium similar to groundwaters and coastal waters of other volcanic islands in the Pacific. However, the basaltic island REE characteristics (with the exception of elevated Y/Ho ratios) are lost during transport to the Manihiki Plateau within surface waters that also exhibit highly radiogenic Nd isotope signatures. Our new data demonstrate that REE concentrations are enriched in Tahitian coastal water, but without multidimensional sampling, basaltic island Nd flux estimates range over orders of magnitude from relatively small to globally significant. Antarctic Intermediate Water (AAIW) loses its characteristic Nd isotopic signature (-6 to-9) around the Manihiki Plateau as a consequence of mixing with South Equatorial Pacific Intermediate Water (SEqPIW), which shows more positive values (-1 to -2). However, an additional Nd input/exchange along the pathway of AAIW, eventually originating from the volcanic Society, Tuamotu and Tubuai Islands (including Tahiti), is indicated by an offset from the mixing array of AAIW and SEqPIW to more radiogenic Nd isotope compositions

Reduced admixture of North Atlantic Deep Water to the deep central South Pacific during the last two glacial periods

Key Points: • Little deep water circulation changes in the past 240,000 years in the central South Pacific • Reduced North Atlantic Deep Water admixture during glacials to the Southern Ocean • South Pacific lithogenic material mainly sourced from SE Australia and South New Zealand The South Pacific is a sensitive location for the variability of the global oceanic thermohaline circulation given that deep waters from the Atlantic Ocean, the Southern Ocean, and the Pacific basin are exchanged. Here we reconstruct the deep-water circulation of the central South Pacific for the last two glacial cycles (from 240,000 years ago to the Holocene) based on radiogenic neodymium (Nd) and lead (Pb) isotope records complemented by benthic stable carbon data obtained from two sediment cores located on the flanks of the East Pacific Rise. The records show small but consistent glacial/interglacial changes in all three isotopic systems with interglacial average values of -5.8 and 18.757 for εNd and 206Pb/204Pb, respectively, whereas glacial averages are -5.3 and 18.744. Comparison of this variability of Circumpolar Deep Water (CDW) to previously published records along the pathway of the global thermohaline circulation is consistent with reduced admixture of North Atlantic Deep Water (NADW) to CDW during cold stages. The absolute values and amplitudes of the benthic δ13C variations are essentially indistinguishable from other records of the Southern Hemisphere and confirm that the low central South Pacific sedimentation rates did not result in a significant reduction of the amplitude of any of the measured proxies. In addition, the combined detrital Nd and strontium (87Sr/86Sr) isotope signatures imply that Australian and New Zealand dust has remained the principal contributor of lithogenic material to the central South Pacific