Calvert Stephen E.

No Thumbnail Available
Last Name
Calvert
First Name
Stephen E.
ORCID

Filters

2004 - 2005 2
2
2
Reset filters

Settings

Search Results

Now showing 1 - 2 of 2
  • Article
    Glacial-interglacial modulation of the marine nitrogen cycle by high-latitude O2 supply to the global thermocline
    (American Geophysical Union, 2004-10-16) Galbraith, Eric D. ; Kienast, Markus ; Pedersen, Thomas F. ; Calvert, Stephen E.
    An analysis of sedimentary nitrogen isotope records compiled from widely distributed marine environments emphasizes the global synchrony of denitrification changes and provides evidence for a strong temporal coupling of these variations to changes in nitrogen fixation as previously inferred. We explain the global coherence of these records by a simple physical control on the flux of dissolved oxygen to suboxic zones and the coupling to fixation via the supply of phosphorus to diazotrophs in suitable environments. According to our hypothesis, lower glacial-stage sea surface temperature increased oxygen solubility, while stronger winds in high-latitude regions enhanced the rate of thermocline ventilation. The resultant colder, rapidly flushed thermocline lessened the spatial extent of denitrification and, consequently, N fixation. During warm periods, sluggish circulation of warmer, less oxygen rich thermocline waters caused expansion of denitrification zones and a concomitant increase in N fixation. Local fluctuations in export productivity would have modulated this global signal.
  • Article
    On the sedimentological origin of down-core variations of bulk sedimentary nitrogen isotope ratios
    (American Geophysical Union, 2005-05-27) Kienast, Markus ; Higginson, M. J. ; Mollenhauer, Gesine ; Eglinton, Timothy I. ; Chen, Min-Te ; Calvert, Stephen E.
    The bulk sedimentary nitrogen isotopic composition of two cores from nearby sites on the northern slope of the South China Sea (Site 17940 and Ocean Drilling Program (ODP) Site 1144) differs by up to >2‰ during the last glacial period. Given their close proximity, both core sites are located in the same biogeographic zone and nutrient regime, and it is thus unlikely that this offset is due to a true gradient in surface ocean conditions. In an attempt to resolve this offset, we have investigated the possible effects of two sedimentological parameters that can affect bulk sedimentary δ15N, namely, the variable contribution of inorganic N to bulk N in the sediment and the grain-size dependence of bulk δ15N. We find that neither effect, singly or in combination, is sufficient to explain the significant δ15N offset between the two down-core records. By elimination the most likely explanation for the observed discrepancy is a different origin of both the organic and inorganic nitrogen at each site. This study adds to the growing body of evidence highlighting the complex nature and origin of the sedimentary components in sediment drifts, such as ODP Site 1144.