Water column

About: Water column is a research topic. Over the lifetime, 13706 publications have been published within this topic receiving 496626 citations.

Absence of deep-water formation in the Labrador Sea during the last interglacial period

Abstract: The two main constituent water masses of the deep North Atlantic Ocean—North Atlantic Deep Water at the bottom and Labrador Sea Water at an intermediate level—are currently formed in the Nordic seas and the Labrador Sea, respectively1. The rate of formation of these two water masses tightly governs the strength of the global ocean circulation and the associated heat transport across the North Atlantic Ocean2. Numerical simulations have suggested a possible shut-down of Labrador Sea Water formation as a consequence of global warming3. Here we use micropalaeontological data and stable isotope measurements in both planktonic and benthic foraminifera from deep Labrador Sea cores to investigate the density structure of the water column during the last interglacial period, which was thought to be about 2 °C warmer than present4. Our results indicate that today's stratification between Labrador Sea Water and North Atlantic Deep Water never developed during the last interglacial period. Instead, a buoyant surface layer was present above a single water mass originating from the Nordic seas. Thus the present situation, with an active site of intermediate-water formation in the Labrador Sea, which settled some 7,000 years ago, has no analogue throughout the last climate cycle.

An extensive bloom of the N2-fixing cyanobacterium Trichodesmium erythraeum in the central Arabian Sea

Abstract: LVe encountered an extensive surface bloom of the N, fixing cyanobactenum Trichodesrniurn erythraeum in the central basin of the Arabian Sea during the spring ~nter-n~onsoon of 1995. The bloom, which occurred dunng a penod of calm winds and relatively high atmospher~c iron content, was metabollcally active. Carbon fixation by the bloom represented about one-quarter of water column primary productivity while input by h:: flxation could account for a major fraction of the estimated 'new' N demand of pnmary production. Isotopic measurements of the N in surface suspended material confirmed a direct contribution of N, fixation to the organic nltrogen pools of the upper water column. Retrospective analysis of NOAA-12 AVHRR imagery indicated that blooms covered up to 2 X 106 km2, or 20% of the Arabian Sea surface, during the period from 22 to 27 May 1995. In addition to their biogeochemical impact, surface blooms of this extent may have secondary effects on sea surface albedo and light penetration as well as heat and gas exchange across the air-sea interface. A preliminary extrapolation based on our observed, non-bloom rates of N, fixation from our limited sampling in the spring intermonsoon, including a conservative estimate of the input by blooms, suggest N2 fixation may account for an input of about 1 Tg N yr-I This is substantial, but relatively minor compared to current estimates of the removal of N through denitrification in the basin. However, N2 fixation may also occur in the central basin through the mild winter monsoon, be considerably greater during the fall intermonsoon than we observed during the spring intermonsoon, and may also occur at higher levels in the chronically oligotrophic southern basin. Ongoing satellite observations will help to determine more accurately the distribution and density of Trichodesmium in this and other tropical oceanic basins, as well as resolving the actual frequency and duration of bloom occurrence.