Santschi Peter H.

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Santschi
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Peter H.
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  • Article
    Intercalibration studies of short-lived thorium-234 in the water column and marine particles
    (Association for the Sciences of Limnology and Oceanography, 2012-09) Maiti, Kanchan ; Buesseler, Ken O. ; Pike, Steven M. ; Benitez-Nelson, Claudia R. ; Cai, Pinghe ; Chen, Weifang ; Cochran, Kirk ; Dai, Minhan ; Dehairs, Frank ; Gasser, Beat ; Kelly, Roger P. ; Masqué, Pere ; Miller, Lisa A. ; Miquel, Juan Carlos ; Moran, S. Bradley ; Morris, Paul J. ; Peine, Florian ; Planchon, Frederic ; Renfro, Alisha A. ; Rutgers van der Loeff, Michiel M. ; Santschi, Peter H. ; Turnewitsch, Robert ; Waples, James T. ; Xu, Chen
    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.
  • Preprint
    Thorium speciation in seawater
    ( 2005-07) Santschi, Peter H. ; Murray, James W. ; Baskaran, Mark ; Benitez-Nelson, Claudia R. ; Guo, L. D. ; Hung, C.-C. ; Lamborg, Carl H. ; Moran, S. Bradley ; Passow, Uta ; Roy-Barman, Matthieu
    Since the 1960’s, thorium isotopes occupy a special place in the oceanographer’s toolbox as tracers for determining rates and mechanisms of oceanic scavenging, particle dynamics, and carbon fluxes. Due to their unique and constant production rates from soluble parent nuclides of uranium and radium, their disequilibrium can be used to calculate rates and time scales of sinking particles. In addition, by ratio-ing particulate 234Th (as well, in principle, other Thnuclides) to carbon (and other elements), and linking this ratio to the parent-daughter disequilibrium in the water column, it is possible to calculate fluxes of carbon and other elements. Most of these applications are possible with little knowledge of the dissolved chemical properties of thorium, other than its oxidation state (IV) and tendency to strongly sorb to surfaces, i.e., its “particle- or surface-activity”. However, the use of any tracer is hindered by a lack of knowledge of its chemical properties. Recent observations in the variability of carbon to 234Th ratios in different particle types, as well as of associations of Th(IV) with various marine organic biomolecules has led to the need for a review of current knowledge and what future endeavors should be taken to understand the marine chemistry of thorium.
  • Preprint
    An assessment of particulate organic carbon to thorium-234 ratios in the ocean and their impact on the application of 234Th as a POC flux proxy
    ( 2005-06-18) Buesseler, Ken O. ; Benitez-Nelson, Claudia R. ; Burd, Adrian B. ; Charette, Matthew A. ; Cochran, J. Kirk ; Coppola, L. ; Fisher, Nicholas S. ; Fowler, Scott W. ; Gardner, Wilford D. ; Guo, L. D. ; Gustafsson, Orjan ; Lamborg, Carl H. ; Masqué, Pere ; Miquel, Juan Carlos ; Passow, Uta ; Santschi, Peter H. ; Savoye, Nicolas ; Stewart, G. ; Trull, Thomas W.
    Thorium-234 is increasingly used as a tracer of ocean particle flux, primarily as a means to estimate particulate organic carbon export from the surface ocean. This requires determination of both the 234Th activity distribution (in order to calculate 234Th fluxes) and an estimate of the C/234Th ratio on sinking particles, to empirically derive C fluxes. In reviewing C/234Th variability, results obtained using a single sampling method show the most predictable behavior. For example, in most studies that employ in situ pumps to collect size fractionated particles, C/234Th either increases or is relatively invariant with increasing particle size (size classes >1 to 100’s μm). Observations also suggest that C/234Th decreases with depth and can vary significantly between regions (highest in blooms of large diatoms and highly productive coastal settings). Comparisons of C fluxes derived from 234Th show good agreement with independent estimates of C flux, including mass balances of C and nutrients over appropriate space and time scales (within factors of 2-3). We recommend sampling for C/234Th from a standard depth of 100 m, or at least one depth below the mixed layer using either large volume size fractionated filtration to capture the rarer large particles, or a sediment trap or other device to collect sinking particles. We also recommend collection of multiple 234Th profiles and C/234Th samples during the course of longer observation periods to better sample temporal variations in both 234Th flux and the characteristic of sinking particles. We are encouraged by new technologies which are optimized to more reliably sample truly settling particles, and expect the utility of this tracer to increase, not just for upper ocean C fluxes but for other elements and processes deeper in the water column.