Welcome to the University of Chicago Melville Cruise website!
We will set sail from San Diego on September 1, 2012 under the guidance of chief scientists Clark Alexander and Susan Kidwell.
Work in the last 20 years on seafloor grab samples has established that molluscan death assemblages from surface sediments can include specimens that are thousands to tens of thousands of years old, especially on open continental shelves. These assemblages are nonetheless strongly dominated by individuals contributed during the last few decades or last century, and thus their temporal resolution for paleoecological and environmental inference (e.g., proxy isotopic data) is much finer than suspected from maximum shell ages. But to what extent do skeletal assemblages retain this temporal resolution - the shape of the shell-age frequency distribution (AFD) observed in the mixed-layer - as they transit into deeper, more permanent layers of the Holocene sedimentary record, where they are no longer refreshed by new shell input and are subject to different diagenetic regimes? What are the relative roles of intrinsic (specific to shell type, functional group) and extrinsic (environmental) factors on the preservation of temporal resolution? What are the net biasing effects on the sedimentary capture of geohistorical information, relevant to evaluating both the status of modern day ecosystems and older stratigraphic records?
Our project leverages extensive prior knowledge of benthic animals, environmental conditions, and the natural and cultural (pollution) history of the Southern California Bight to quantify, for the first time, (1) down-core variation in the temporal resolution of carbonate shell assemblages and (2) spatial and down-core trends as a function of physical and biogenic reworking rates, sediment composition (grain size, TOC, porewaters), and net burial rates. It will thus provide data to (3) model the dynamics of skeletal fossilization over the critical initial few millennia of sediment accumulation and (4) evaluate the compositional fidelity of very young records. At three (possibly four) positions along the Southern California continental margin, we will occupy stations in 50 and 75 m water depths on the continental shelf. At each station we will take two large box cores (max 50 cm penetration) and multiple 3-4 m long cores (Kasten or vibracores) in order to recover the large sedimentary volumes needed to insure sufficient molluscan material. Stations target a variety of seafloor types from abundant sand and shell gravel (Orange County) to those dominated by mud (Palos Verdes and Malibu); muddy sites are arrayed along a legacy gradient of sediment toxicity from DDT, which has stunted deep-burrowing infauna locally. At each station, post-cruise radiocarbon-calibrated amino-acid racemization will be used to establish shell ages for millennial timescale chronology in long cores and Pb-210/Cs-137/Th-234 will be used to establish profiles of sediment reworking and accumulation rates. The upper increments of cores will be processed on board for dead macrobenthic shells, with any living specimens archived for future DNA work. To determine offshore sediment transport rates and bathymetric gradients in sediment mixing using short-lived radionuclides, we will collect a single box core in 200 m water depth at each of the shelf-coring areas. We also plan 3 pairs of box cores from the continental margin between San Diego and Orange County, one on the shelf (50 m) and one on the slope (200 m), during our transits.
Relevant to fossilization dynamics and the capture of ecological and environmental information, we have recruited additional scientists to work within our framework to evaluate: (a) carbonate saturation of pore-waters at each shelf station (for shell-dissolution kinetics; collaborative with R. Aller); (b) a small-volume method of pCO2 determination (C. Hintz); (c) spatial & down-core isotopic variation in bulk sediments and molluscan shells, especially of chemosymbiotic species (M. Villarosa, A. Colman); (d) diagenetic alteration of proteins in marine sediments, including shell fragments, for biomarker development (J. Waldbauer); (e) persistence and rates of phosphatase activity following microbial cell lysis (A. Mine); and (f) planktonic microbial genomic diversity in overlying water (M. Coleman, J. Waldbauer). Cruise work will thus include specialized subsampling of cores, archiving of one complete core at each shelf station, a CTD cast at each station, and on-board stabilization and/or lab processing of water samples for carbonate chemistry, stable isotopes, and microbial content. If time permits between cores, additional CTD casts will be taken to quantify microbial activity. We will provide a platform of research for four junior faculty and cruise experience for nine undergraduate and six PhD students from three institutions (University of Chicago, Savannah State University, Scripps).