Description: Climate conditions in the westernmost Mediterranean (Alboran Sea basin) over the last two millennia have been reconstructed through integration of molecular proxies applied for the first time in this region at such high resolution. Two temperature proxies, one based on isoprenoid membrane lipids of marine Thaumarchaeota (TEXH86-tetraether index of compounds consisting of 86 carbons) and the other on alkenones produced by haptophytes (UK'37 ratio) were applied to reconstruct sea surface temperature (SST). Both records reveal a progressive long term decline in SST over the last two millennia and an increased rate of warming during the second half of the twentieth century. This is in accord with previous temperature reconstructions for the Northern Hemisphere. TEXH86 temperature values are higher than those inferred from UK'37, probably due to differences in the bloom season of haptophytes and Thaumarchaeota, and reflect summer SST. The branched vs. isoprenoid tetraether index (BIT index) suggests a low contribution of soil organic matter (OM) to the sedimentary OM. The stable carbon isotopic composition of long chain n-alkanes indicates a predominant C3 plant contribution, with no major change in vegetation over the last 2000 yr. The distribution of long chain 1,14-diols (most likely sourced by Proboscia species in this setting) provided insight into variation in upwelling conditions during the last 2000 yr and depicts a correlation with the North Atlantic Oscillation (NAO) index, providing evidence of enhanced wind induced upwelling during periods of a persistent positive mode of the NAO.

Description: The strength of the Atlantic Meridional Overturning Circulation (AMOC) is a major factor in the global thermohaline circulation system. Because of this central role of AMOC in the global climate system, it is important to have reliable tools to reconstruct the strength of deep water ventilation in general and of AMOC in particular during major phases of past climate variability. The basin-wide Atlantic 231Pa/230Th signature in sediment archives has been identified as a powerful tracer for the intensity of the AMOC in the past. The information on ocean circulation arises from an overall higher export rate of 231Pa over 230Th within NADW from the entire Atlantic basin to the Southern Ocean due to the somewhat lower particle reactivity of 231Pa. A fundamental complication for reliable applicability of this tool arises from the fact that there is a severe lack of data of these isotopes in the critical areas of the modern AMOC. Only very few 230Th and 231Pa profiles in the Labrador Sea, in the NE Atlantic, and in the tropical and South Atlantic have been published [1, 2, 3, 4]. To remove this gap, 14 water column profiles were sampled under trace metal clean conditions during the expedition of RV Pelagia (GEOTRACES Cruise GA02) on a transect following the North Atlantic deep water (NADW). At all stations the deepest sample was collected within the bottom nepheloid layer, providing information on the latest stage of signal development in the water column. Here we will present the first results on 5 profiles of dissolved 230Th and 231Pa along NADW between 15 and 55°N including also the crossover station Bermuda Atlantic Time-Series (BATS). With this study we aim to provide missing information of the factors controlling signal generation in order to answer the questions: What is the 231Pa/230Th isotope composition of the main water masses of the AMOC and how do the 230Th and 231Pa activities in NADW evolve on its way south and east? Can they be explained by ventilation or are there other controlling factors to identify such as the composition of suspended particles? [1] Moran et al. (2002) Earth Planet. Sci. Lett. 203, 999-1014. [2] Moran et al. (1997) Earth Planet. Sci. Lett. 150, 151-160. [3] Moran et al. (1995) Geophys. Res. Lett. 22, 2589-2592. [4] Vogler et al. (1998) Earth Planet. Sci. Lett. 156, 61-74

Description: In this study the first comprehensive dataset of 236U in the North Atlantic Ocean is presented. 236U/238U ratios and 236U concentrations are determined in about 70 seawater samples collected from 9 depth profiles along a transect through the Northwest Atlantic Ocean during the first two legs of the GEOTRACES cruise GA02 in 2010. The cruise track was designed to follow the path of the North Atlantic Deep Water (NADW) from its formation region to the deep parts of the western Atlantic Ocean basins. Our results show that there is a strong North to South and surface to deep ocean gradient spanning a broad range of 236U/238U, from (44±15)x10-12 to (1477±91)x10-12 that correlates well with the distribution of water masses in this region. All measured ratios are above the theoretical pre-anthropogenic levels of 10-13 to 10-14, showing that the whole transect is dominated by anthropogenic 236U. Based on the calculated 236U inventories the 236U inputs of global fallout and direct releases from nuclear reprocessing plants could be constrained to a range from 1000 to 1400 kg and from 115 to 250 kg of 236U, from the two sources, respectively. Our results show that the 236U/238U atomic ratio can be used as a marker of North Atlantic water masses: with high ratios (〉 1000x10-12) characterizing northern water masses (LSW and DSOW); and lowest ratios (50-400 x10-12) corresponding to the increasing influence of AABW and AAIW.

Description: Author Posting. © Association for the Sciences of Limnology and Oceanography, 2012. This article is posted here by permission of Association for the Sciences of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography: Methods 10 (2012): 631-644, doi:10.4319/lom.2012.10.631.

Description: Intercomparision of 234Th measurements in both water and particulate samples was carried out between 15 laboratories worldwide, as a part of GEOTRACES inter-calibration program. Particulate samples from four different stations namely BATS (both shallow and deep) and shelf station (shallow) in Atlantic and SAFE (both shallow and deep) and Santa Barbara station (shallow) in Pacific were used in the effort. Particulate intercalibration results indicate good agreement between all the participating labs with data from all labs falling within the 95% confidence interval around the mean for most instances. Filter type experiments indicate no significant differences in 234Th activities between filter types and pore sizes (0.2-0.8 μm). The only exception are the quartz filters, which are associated with 10% to 20% higher 234Th activities attributed to sorption of dissolved 234Th. Flow rate experiments showed a trend of decreasing 234Th activities with increasing flow rates (2-9 L min-1) for 〉 51 μm size particles, indicating particle loss during the pumping process. No change in 234Th activities on small particles was observed with increasing flow-rates. 234Th intercalibration results from deep water samples at SAFe station indicate a variability of 〈 3% amongst labs while dissolved 234Th data from surface water at Santa Barbara Station show a less robust agreement, possibly due to the loss of 234Th from decay and large in-growth corrections as a result of long gap between sample collection and processing.

Description: This research is funded by NSF Chemical Oceanography program. LM will like to thank Fisheries and Oceans Canada for support. PM is supported through ICREA Academia funded by Generalitat de Catalunya. The International Atomic Energy Agency is grateful to the Government of the Principality of Monaco for the support provided to its Environment Laboratories.

Description: © The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeosciences 10 (2013): 3649-3659, doi:10.5194/bg-10-3649-2013.

Description: The impact of the earthquake and tsunami on the east coast of Japan on 11 March 2011 caused a loss of power at the Fukushima Dai-ichi nuclear power plant (NPP) that resulted in one of the most important releases of artificial radioactivity into the environment. Although several works were devoted to evaluating the atmospheric dispersion of radionuclides, the impact of the discharges to the ocean has been less investigated. Here we evaluate the distribution of Fukushima-derived 90Sr (n = 57) and 89Sr (n = 19) throughout waters 30–600 km offshore in June 2011. Concentrations of 90Sr and 89Sr in both surface waters and shallow profiles ranged from 0.8 ± 0.2 to 85 ± 3 Bq m−3 and from 19 ± 6 to 265 ± 74 Bq m−3, respectively. Because of its short half-life, all measured 89Sr was due to the accident, while the 90Sr concentrations can be compared to the background levels in the Pacific Ocean of about 1.2 Bq m−3. Fukushima-derived radiostrontium was mainly detected north of Kuroshio Current, as this was acting as a southern boundary for transport. The highest activities were associated with near-shore eddies, and larger inventories were found in the closest stations to Fukushima NPP. The data evidence a major influence of direct liquid discharges of radiostrontium compared to the atmospheric deposition. Existing 137Cs data reported from the same samples allowed us to establish a 90Sr / 137Cs ratio of 0.0256 ± 0.0006 in seawater off Fukushima, being significantly different than that of the global atmospheric fallout (i.e., 0.63) and may be used in future studies to track waters coming from the east coast of Japan. Liquid discharges of 90Sr to the ocean were estimated, resulting in an inventory of 53 ± 1 TBq of 90Sr in the inshore study area in June 2011 and total releases of 90Sr ranging from 90 to 900 TBq, depending upon the reported estimates of 137Cs releases that are considered.

Description: We gratefully acknowledge funding from the Gordon and Betty Moore Foundation with additional support from the Chemical Oceanography Program of the US National Science Foundation and the Woods Hole Oceanographic Institution. P. M. acknowledges the ICREA Academia award, funded by the Generalitat de Catalunya. Support for this project was provided by the Ministerio de Ciencia y Educaci´on of Spain (CTM2011-15152-E).