Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Program Information
• Funding

Seawater samples were collected in the GEOTRACES GA02 Cruise Leg 3 from Punta Arenas (Chile) to Las Palmas (Spain), March-April 2011. 5-10 L of seawater were collected depending on the sample depth and stored in a cubitainer for each sample. The samples were filtered using 0.2 μm Sartobran cartridges and acidified using ultrapure Seastar hydrochloric acid (HCl) to pH ~ 2 shortly after sampling.

We preconcentrated rare earth elements from seawater using C18 cartridges (Waters Corp., Sep-Pak classic C18 cartridge 360 mg 55-105 μm) loaded with complexing agent of 2-ethylhexyl hydrogen phosphate (HDEHP) and 2-ethylhexyl dihydrogen phosphate (H2MEHP), which was first proposed by Shabani et al. (1992). In this study, we followed the method by Jeandel et al. (1998), Lacan and Jeandel (2004), Pahnke et al. (2012). Specifically, C18 cartridges were first cleaned in a 0.5N HCl bath overnight, then 10 mL of 6N HCl were introduced through the cartridge, and then they were flushed with > 500 mL of Milli-Q water. Cartridges were in a neutral environment (MilliQ® water) after cleaning. For each 5 L sample, 300 μL of complexing agent HDEHP/H2MEHP was loaded on a clean cartridge. Seawater samples were adjusted to pH ~ 3.5 by adding Optima® ammonium hydroxide. The seawater samples were pumped through the cartridges at 20mL/min by a peristaltic pump. Afterward, the cartridges were eluted with 10 mL of 0.01N HCl to remove barium. After barium elution, the cartridges were eluted with 35 mL of 6N hydrochloric acid at 10 mL/min by the peristaltic pump to collect the REEs. The REEs were dried and further purified by Eichrom RE-spec column chemistry. Nd fractions were extracted from REEs by LN-spec column chemistry.

The Nd isotopic ratios were measured on MC-ICP-MS (a ThermoScientific Neptune-Plus®) at Lamont-Doherty Earth Observatory of Columbia University. Nd fractions from column chemistry were dried by evaporation and dissolved in 600-800 μL of 3% nitric acid (HNO3). The international Nd standard JNdi-1 was analyzed between every sample and the average of the standards is reported relative to the recommended value of 143Nd/144Nd = 0.512115 (Tanaka, 2000). For different concentrations of JNdi-1, the measured 143Nd/144Nd isotopic ratios were 0.512079 ± 0.000014 (2σ, n = 126) for 20ppb solutions, 0.512079 ± 0.000013 (2σ, n = 190) for 15 ppb solutions, and 0.512058 ± 0.000022 (2σ, n = 37) for 10ppb solutions. Nd isotopic ratios were normalized to 146Nd/144Nd = 0.7219 for mass fractionation.

We are awaiting the results of Nd isotopic ratios at the crossover station (~40°S and ~42°W) from University of Cambridge.

The Nd isotopic composition is expressed as εNd = [(143Nd/144Ndsample/143Nd/144NdCHUR) - 1] × 104, where 143Nd/144NdCHUR = 0.512638 (Jacobsen and Wasserburg, 1980).

BCO-DMO Processing notes:
- added conventional header with dataset name, PI name, version date
- modified parameter names to conform with BCO-DMO naming conventions
- blank values replaced with no data value 'nd'
- replaced spaces with underscores
- converted BTL_ISO_DateTime_UTC to standard format
- reduced decimals of Nd values

Extracted from the NSF award abstract:

Neodymium (Nd) isotopes are increasingly used in paleoceanographic studies as "quasi-conservative" water mass tracers. However, the limitations of this proxy are not yet fully understood. The proposed work aims to address this uncertainty by critically evaluating the behavior of Nd isotopes as tracers of water mass mixing. The project, led by researchers at Columbia University's Lamont-Doherty Earth Observatory, will analyze in-hand seawater and surface sediment samples collected along a meridional transect in the southwest Atlantic (0 to 50 degrees S) during a GEOTRACES cruise. The sample suite will be used to test 1) whether Nd isotope ratios deviate from expected values for mixing along circulation transport paths, 2) whether Nd isotopes behave quasi-conservatively away from continental margins, 3) whether seafloor features (e.g., continental shelf, volcanic seamounts) add significant external Nd to the system, and 4) whether the Southern Hemisphere wind zones impact Nd isotope values through aeolian deposition. The relationship between Nd isotopes in authigenic surface sediments and those in the overlying seawater will be calibrated for the first time.

By testing an emerging tool in the study of past ocean dynamics, this research will enable a more accurate understanding of changes in the ocean-climate system. The project will support an early-career researcher and a graduate student. Undergraduate students will be involved through an NSF-supported summer internship program at LDEO.

GEOTRACES is a SCOR sponsored program; and funding for program infrastructure development is provided by the U.S. National Science Foundation.

GEOTRACES gained momentum following a special symposium, S02: Biogeochemical cycling of trace elements and isotopes in the ocean and applications to constrain contemporary marine processes (GEOSECS II), at a 2003 Goldschmidt meeting convened in Japan. The GEOSECS II acronym referred to the Geochemical Ocean Section Studies To determine full water column distributions of selected trace elements and isotopes, including their concentration, chemical speciation, and physical form, along a sufficient number of sections in each ocean basin to establish the principal relationships between these distributions and with more traditional hydrographic parameters;

* To evaluate the sources, sinks, and internal cycling of these species and thereby characterize more completely the physical, chemical and biological processes regulating their distributions, and the sensitivity of these processes to global change; and

* To understand the processes that control the concentrations of geochemical species used for proxies of the past environment, both in the water column and in the substrates that reflect the water column.

GEOTRACES will be global in scope, consisting of ocean sections complemented by regional process studies. Sections and process studies will combine fieldwork, laboratory experiments and modelling. Beyond realizing the scientific objectives identified above, a natural outcome of this work will be to build a community of marine scientists who understand the processes regulating trace element cycles sufficiently well to exploit this knowledge reliably in future interdisciplinary studies.

Expand "Projects" below for information about and data resulting from individual US GEOTRACES research projects.