HR: 17:00h
AN: PP52C-05    [PDF]
TI: On the potential sedimentological origin of downcore variations of bulk sedimentary $\delta^{15}$N
AU: * Kienast, M
EM: mkienast@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Higginson, M J
EM: mhigginson@UMassD.Edu
AF: The School for Marine Science & Technology, University of Massachusetts at Dartmouth, New Bedford, MA 02744 United States
AU: Mollenhauer, G
EM: gmollenhauer@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Eglinton, T I
EM: teglinton@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Calvert, S E
EM: calvert@eos.ubc.ca
AF: Oceanography, Dept. EOS, University of British Columbia, Vancouver, BC V6T 1Z4 Canada
AB: ODP site 1144 is located less than ten kilometers from core site SONNE 17940 on the continental slope of the northern South China Sea (SCS). Despite their proximity, the sedimentary nitrogen isotope records are distinctly different at both sites, with glacial-interglacial variations of ca. 1 permil at site 17940, and up to 4 permil at site ODP1144. Here we explore the potential origin of these differences in the $\delta^{15}$N records, focussing on three aspects of the variable sedimentology at both sites on glacial-interglacial timescales. 1) Based on major element contents (Si/Al and Zr/Al ratios), glacial sediments at site ODP1144 are significantly finer-grained than at site 17940. As evident from a suite of samples from the SCS, finer-grained sediments are associated with higher $\delta^{15}$N values, thus contributing to the offset in the $\delta^{15}$N records between both sites. 2) Sediments at site ODP1144 contain lower amounts of potassium, and, by inferrence, ammonium, which substitutes for potassium in K-bearing minerals. Given the low $\delta^{15}$N of ammonium fixed in clay minerals this difference in mineralogy further contributes to lower glacial $\delta^{15}$N values at site 17940 compared to ODP1144. 3) We will also be presenting radiocarbon dates of total organic carbon (TOC), in an attempt to elucidate the different origin and sedimentological history of TOC at both sites. Sediments found at both ODP1144 and 17940 originate from an area affected by eustatic sediment redistribution, gravity- and fluvially-driven sediment mobilization from the broad northern SCS continental shelf. Rare earth element analyses (Shao et al., 2001) indicate that a significant part of the detrital material at ODP1144 originates from Taiwan, transported through the Penghu channel to the coring site in the northern SCS, and is not representative of the vertical particle flux to the sea floor. Sediment redistribution therefore potentially affects downcore variations in bulk sedimentary $\delta^{15}$N, and cautions the interpretation of a single downcore record with respect to local/regional $\delta^{15}$N variations in the past, and indeed with respect to other sedimentary proxies.
DE: 1040 Isotopic composition/chemistry
DE: 3022 Marine sediments--processes and transport
DE: 4243 Marginal and semienclosed seas
DE: 4267 Paleoceanography
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting