Abstract
Abstract. Surface waters (0–200 m) of the western tropical South Pacific (WTSP) were sampled along a longitudinal 4000 km transect (OUTPACE cruise, DOI: 10.17600/15000900) during the austral summer (stratified) period (18 February to 3 April 2015) between the Melanesian Archipelago (MA) and the western part of the SP gyre (WGY). Two distinct areas were considered for the MA, the western MA (WMA), and the eastern MA (EMA). The main carbon (C), nitrogen (N), and phosphorus (P) pools and fluxes provide a basis for the characterization of the expected trend from oligotrophy to ultra-oligotrophy, and the building of first-order budgets at the daily and seasonal timescales (using climatology). Sea surface chlorophyll a well reflected the expected oligotrophic gradient with higher values obtained at WMA, lower values at WGY, and intermediate values at EMA. As expected, the euphotic zone depth, the deep chlorophyll maximum, and nitracline depth deepen from west to east. Nevertheless, phosphaclines and nitraclines did not match. The decoupling between phosphacline and nitracline depths in the MA allows for excess P to be locally provided in the upper water by winter mixing. We found a significant biological “soft tissue” carbon pump in the MA sustained almost exclusively by dinitrogen (N2) fixation and essentially controlled by phosphate availability in this iron-rich environment. The MA appears to be a net sink for atmospheric CO2, while the WGY is in quasi-steady state. We suggest that the necessary excess P, allowing the success of nitrogen fixers and subsequent carbon production and export, is mainly brought to the upper surface by local deep winter convection at an annual timescale rather than by surface circulation. While the origin of the decoupling between phosphacline and nitracline remains uncertain, the direct link between local P upper water enrichment, N2 fixation, and organic carbon production and export, offers a possible shorter timescale than previously thought between N input by N2 fixation and carbon export. The low iron availability in the SP gyre and P availability in the MA during the stratified period may appear as the ultimate control of N input by N2 fixation. Because of the huge volume of water to consider, and because the SP Ocean is the place of intense denitrification in the east (N sink) and N2 fixation in the west (N source), precise seasonal C, N, P, and iron (Fe) budgets would be of prime interest to understand the efficiency, at the present time and in the future, of the oceanic biological carbon pump.
Highlights
The oceanic biological carbon pump corresponds to the transfer of carbon from the upper surface to the ocean interior by biological processes, greatly influencing atmospheric CO2 concentration and the earth’s climate
Expected seasonal upper surface temperature variations calculated from the differences between temperature at the surface and at 70 m depth were 3.6 ± 0.6, 4.5 ± 1.2, and 3.4 ± 1.3 ◦C for western MA (WMA), eastern Melanesian Archipelago (EMA), and western gyre (WGY), respectively, agreed relatively well with Sea surface temperature (SST) variations observed between July 2014 and March 2015 of 3.9 ± 0.5, 4.2 ± 1.4, and 2.6 ± 0.6 ◦C (Fig. 2b, e, h)
We established a relatively good comparison between Chlorophyll a (Chl a) measured at 70 m depth during OUTPACE of 0.217 ± 0.092, 0.091 ± 0.012, and 0.046 ± 0.010 mg m−3 for WMA, EMA, and WGY, respectively (Fig. 3f), and sea surface Chl a (SSChl a) obtained during the deeper mixing of 0.173 ± 0.005, 0.121 ± 0.023, and 0.042 ± 0.002 mg m−3 for WMA, EMA, and WGY, respecwww.biogeosciences.net/15/2961/2018/
Summary
The oceanic biological carbon pump corresponds to the transfer of carbon from the upper surface to the ocean interior by biological processes, greatly influencing atmospheric CO2 concentration and the earth’s climate. It is a highly ranked priority in current research in oceanography (Burd et al, 2016). While the observed changes in N2 fixation and biogeochemical cycling reflect either natural oceanic variability or climate change (Karl et al, 1997; Karl, 2014), the most probable changes for the near future in both N2 fixation and denitrification processes following climate forcing are predicted to be a strengthening control of the carbon cycle by P availability (Moutin et al, 2008)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
