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.

Molina-Kescher, Mario (2014) 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. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 160 pp.

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Abstract

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.

Document Type: Thesis (PhD/ Doctoral thesis)
Thesis Advisor: Frank, Martin and Pahnke, Katharina
Keywords: Deep-water circulation, detrital provenance, South Pacific, Nd isotopes, Sr isotopes, Pb isotopes, Rare Earth Elements
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-P-OZ Paleo-Oceanography
Open Access Journal?: Yes
Date Deposited: 26 Aug 2014 07:12
Last Modified: 26 Aug 2014 07:18
URI: http://oceanrep.geomar.de/id/eprint/25484

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