On the sedimentological origin of down‐core variations of bulk sedimentary nitrogen isotope ratios

Abstract

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.

Spatiotemporal variations of nitrogen isotopic records in the Arabian Sea

“…6). Such a positive deviation can be seen elsewhere in previous reports (Altabet, 1988;Brummer et al, 2002;Kienast et al, 2005) due to prolonged oxygen exposure after deposition (Robinsson et al, 2012) associated with sedimentation rate (Pichevin et al, 2007). Although Cowie et al (2009) found an ambiguous relation between contents of sedimentary organic carbon and oxygen in deep water, they also noticed the appearance of maximum organic carbon contents at the lower boundary of ODZs, where oxygen content was relatively higher.…”

North Pacific-wide spreading of isotopically heavy nitrogen during the last deglaciation: Evidence from the western Pacific

“…These complicated processes that may have led to the unrecognizable patterns of temporal variations of δ 15 N during the last deglaciation have been described previously (Higginson et al, 2003). On the other hand, the terrestrial input from different sources might also affect the δ 15 N record in the SCS (Kienast et al, 2005). By comparison, the relatively simple circulation in the open North Pacific Ocean allows the transmission of the signal across the entire basin from the eastern tropical Pacific (Fig.…”

Nitrogen isotopes in bulk marine sediment: linking seafloor observations with subseafloor records

“…The latter is dominated by clay-bound NH + 4 , and can be significant in areas of high clay input relative to organic matter sedimentation Kienast et al, 2005). Terrestrial organic matter can also be significant in the vicinity of rivers or glacial discharge with high particulate loads (McKay et al, 2004), but tends to be overwhelmed by marine organic matter elsewhere (Altabet, 2006).…”

“…IAEA-N1, -N2, NIST 8548 or local air), and some laboratories supplement these with their own sediment standards. The combusted material is dominated by marine organic matter at most locations, although there is significant contamination by clay-bound inorganic (Kienast et al, 2005) and terrigenous nitrogen ) in some locations. Given the known problems with acidification of samples (Brodie et al, 2011), we flagged samples that had been acidified where possible, though acidification is not always reported.…”

“…The compilation here suggests that, in most of the ocean, spatial averaging by turbulent ocean circulation conspires with temporal averaging over centuries of deposition such that seafloor sediment generally provides a consistent reflection of the surrounding environment on a scale of order of 100 km, in 85 % of cases. This consistency does not mean that the seafloor bulk δ 15 N necessarily reflects exported marine organic δ 15 N, as contamination by terrestrial N occurs in some places (Kienast et al, 2005;Meckler et al, 2011), a concern that must be evaluated on a site-specific basis. .…”