Biogeochemical investigation of the benthic nitrogen cycle in the Arabian Sea off Pakistan

A pronounced deficit of nitrogen (N) in the oxygen minimum zone (OMZ) of the Arabian Sea suggests the occurrence of heavy N-loss that is commonly attributed to pelagic processes. However, the OMZ water is in direct contact with sediments on three sides of the basin. Contribution from benthic N-loss to the total N-loss in the Arabian Sea remains largely unassessed. In October 2007, we sampled the water column and surface sediments along a transect cross-cutting the Arabian Sea OMZ at the Pakistan continental margin, covering a range of station depths from 360 to 1430 m. Benthic denitrification and anammox rates were determined by using 15N-stable isotope pairing experiments. Intact core incubations showed declining rates of total benthic N-loss with water depth from 0.55 to 0.18 mmol N m**-2 day**-1. While denitrification rates measured in slurry incubations decreased from 2.73 to 1.46 mmol N m**-2 day**-1 with water depth, anammox rates increased from 0.21 to 0.89 mmol N m**-2 day**-1. Hence, the contribution from anammox to total benthic N-loss increased from 7% at 360 m to 40% at 1430 m. This trend is further supported by the quantification of cd1-containing nitrite reductase (nirS), the biomarker functional gene encoding for cytochrome cd1-Nir of microorganisms involved in both N-loss processes. Anammox-like nirS genes within the sediments increased in proportion to total nirS gene copies with water depth. Moreover, phylogenetic analyses of NirS revealed different communities of both denitrifying and anammox bacteria between shallow and deep stations. Together, rate measurement and nirS analyses showed that anammox, determined for the first time in the Arabian Sea sediments, is an important benthic N-loss process at the continental margin off Pakistan, especially in the sediments at deeper water depths. Extrapolation from the measured benthic N-loss to all shelf sediments within the basin suggests that benthic N-loss may be responsible for about half of the overall N-loss in the Arabian Sea.

Dinoflagellate cysts abundance and events of ODP Hole 189-1168A sediments

Palynomorphs were studied in samples from Ocean Drilling Program (ODP) Leg 189, Hole 1168A (slope of the western margin of Tasmania; 2463 m water depth). Besides organic-walled dinoflagellate cysts (dinocysts), broad categories of other palynomorphs were quantified in terms of relative abundance. In this contribution, we provide an overview of the early late Eocene-Quaternary dinocyst distribution and illustrate main trends in palynomorph distribution. Dinocyst species throughout Hole 1168A are largely cosmopolitan with important contributions of typical low-latitude taxa and virtual absence of endemic Antarctic taxa. Dinocyst stratigraphic distribution broadly matches that known from the Northern Hemisphere and equatorial regions, although significant differences are noted. Selected potentially biochronostratigraphically useful events are summarized. The distribution of dinocysts in the middle-upper Miocene interval is rather patchy, probably due to prolonged exposure to oxygen. An important general aspect in the dinocyst assemblages is the near absence of Antarctic endemic species and the apparent influence of relatively warm waters throughout the succession at Site 1168. General palynomorph distribution indicates continued deepening from an initial shallow, even restricted, marine setting from late Eocene-Quaternary times. A curious massive influx of small skolochorate acritarchs is recorded throughout the late early-early middle Miocene; the significance of this signal is not yet understood. A general long-term oligotrophic nature of the surface waters influencing Site 1168 is suggested from the low abundance of (proto) peridinioid, presumably heterotrophic, species. The overall dinocyst distribution pattern corresponds to the long-term existence of a Leeuwin-like current influencing the region, including Site 1168, confirming results of earlier studies on other microfossil groups. The occasional influence of colder surface water conditions is, however, also apparent, notably during the late Pliocene-Quaternary, indicating the potential of high-resolution dinocyst analysis for future paleoceanographic studies.