Distributions, trends and inter-annual variability of nutrients along a repeat section through the Weddell Sea (1996–2011)

Abstract

Nutrient data from five repeat sections spanning 1996 to 2011 crossing the Weddell Sea are presented. These measurements have been standardized against the same reference material, yielding an outstanding internal consistency. The generic structure of the Weddell Gyre and its hydrographic features are visible in the nutrient distributions; variability is largest in the Circumpolar Deep Water (CDW) and only minor in other water masses. The distribution of silicate appears to be very powerful for describing water mass processes in the bottom layer. The distribution of nitrite is described in detail for the first time. Opposed to common knowledge, a considerable part of the abyssal Weddell Sea had detectable nitrite concentrations, hinting at biological activity at these depths. Also the bottom water, which definitely exists on a longer-than-seasonal scale, has a nitrite signature, thus challenging the commonly assumed short life-time of nitrite. We infer significant trends of increasing nutrient concentrations in the surface layer, which are attributed mainly to an increasing rate of upwelling of subsurface water over those 15 years. Also in the depth range of the CDW an increasing trend is found, which indicates that the nutrient supply to the gyre may have increased. In the bottom water, silicate exhibits an increasing trend, which is probably caused by a changing composition of the bottom water, i.e. containing more CDW and less surface water.

Variability of nutrients and carbon dioxide in the Antarctic Intermediate Water between 1990 and 2014

“…This is in line with the findings of Tanhua et al (2017) who reported a speed-up of ventilation in the upper intermediate waters of the Atlantic sector of the SO between 1998 and 2012 based on the analysis of transient tracers (SF 6 /CFC-12). The increase of nitrate due to stronger upwelling of nutrient-rich deep water is in accord with Hoppema et al (2015) who reported an increase in nutrient concentration in the surface waters of the Weddell Sea between 1996 and 2011 and linked the changes observed in nutrients to the increase in upwelling. Also, Pardo et al (2017) conclude that biogeochemical change in the Southern Ocean south of Tasmania between 1995 and 2011 supports a scenario of intensification of upwelling caused by an increase in westerly winds.…”

The Weddell Gyre, Southern Ocean: Present Knowledge and Future Challenges

“…The carbon balance in the WG has been perturbed by a strengthening of the subpolar westerlies (Marshall, ; D.W.J. Thompson et al, ) that cause stronger upwelling of carbon and nutrient‐rich subsurface water (Hauck et al, ; Hoppema et al, ). On the one hand, additional CO 2 input from depth can lead to a reduction of CO 2 uptake (le Quéré et al, ).…”

“…On the one hand, additional CO 2 input from depth can lead to a reduction of CO 2 uptake (le Quéré et al, ). On the other hand, the concomitant vertical supply of macronutrients and micronutrients (Hoppema et al, ) could stimulate biological production, leading to more atmospheric CO 2 uptake, thereby moderating the reduction in CO 2 uptake (Hauck et al, ). While the dynamics of the seasonality of the carbon cycle in the WG are roughly understood (Brown et al, ), winter data are scarce (Bakker et al, ; Figure ), and a concerted effort is needed to reduce the uncertainties on annual CO 2 uptake and variability on subseasonal to interannual time scales (Monteiro et al, ) as a benchmark for future change.…”

“…We discuss three characteristics of elemental cycling of the WG, which makes it unique within the global ocean context. (1) The WG is one of the few regions in the global ocean where deep upwelling contributes strongly to the distribution of tracers (Rutgers van der Loeff & Berger, ) and nutrients (Dieckmann et al, ; Hoppema et al, , ; Papadimitriou et al, ). In consequence, many chemical species enriched in intermediate and deep waters can be found at elevated concentrations in surface layers.…”

Meridional circulation across the Antarctic Circumpolar Current serves as a double 231Pa and 230Th trap

“…All model parameter values are listed in Table 264 S2. These three different scavenging depth regimes seem to be related to a combination of different 265 particle fluxes: a component with shallow mineralization that releases 210 Pb at shallow depth, an opal 266 flux that carries 231 Pa to depth where it is released upon opal dissolution (Hoppema et al, 2015), and a 267 third component that is not mineralized and sinks to the seafloor. The shallow release of 210 (Table 2).…”