A rapid transition from ice covered CO2-rich waters to a biologically mediated CO2 sink in the eastern Weddell Gyre

Abstract. Circumpolar Deep Water (CDW), locally called Warm Deep Water (WDW), enters the Weddell Gyre in the southeast, roughly at 25° E to 30° E. In December 2002 and January 2003 we studied the effect of entrainment of WDW on the fugacity of carbon dioxide (fCO2) and dissolved inorganic carbon (DIC) in Weddell Sea surface waters. Ultimately the fCO2 difference across the sea surface drives air-sea fluxes of CO2. Deep CTD sections and surface transects of fCO2 were made along the Prime Meridian, a northwest-southeast section, and along 17° E to 23° E during cruise ANT XX/2 on FS Polarstern. Upward movement and entrainment of WDW into the winter mixed layer had significantly increased DIC and fCO2 below the sea ice along 0° W and 17° E to 23° E, notably in the southern Weddell Gyre. Nonetheless, the ice cover largely prevented outgassing of CO2 to the atmosphere. During and upon melting of the ice, biological activity rapidly reduced surface water fCO2 by up to 100 μatm, thus creating a sink for atmospheric CO2. Despite the tendency of the surfacing WDW to cause CO2 supersaturation, the Weddell Gyre may well be a CO2 sink on an annual basis due to this effective mechanism involving ice cover and ensuing biological fCO2 reduction. Dissolution of calcium carbonate (CaCO3) in melting sea ice may play a minor role in this rapid reduction of surface water fCO2.

[1]  Joint Panel on Oceanographic Tables.  Thermodynamics of the carbon dioxide system in seawater : report , 1987 .

[2]  Andrew G. Dickson,et al.  Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water. Version 2 , 1994 .

[3]  P. Minnett,et al.  The Northeast Water Polynya as an atmospheric CO2 sink: A seasonal rectification hypothesis , 1995 .

[4]  H. Kennedy,et al.  Stratification and the distribution of phytoplankton, nutrients, inorganic carbon, and sulfur in the surface waters of Weddell Sea leads , 2008 .

[5]  A. Weaver,et al.  Response of the global carbon cycle to human‐induced changes in Southern Hemisphere winds , 2007 .

[6]  Michael Schröder,et al.  On the structure and the transport of the eastern Weddell Gyre , 1999 .

[7]  Casper Labuschagne,et al.  Saturation of the Southern Ocean CO2 Sink Due to Recent Climate Change , 2007, Science.

[8]  H. Kennedy,et al.  Experimental evidence for carbonate precipitation and CO2 degassing during sea ice formation , 2004 .

[9]  A. Orsi,et al.  On the meridional extent and fronts of the Antarctic Circumpolar Current , 1995 .

[10]  K. Johnson,et al.  Coulometric total carbon dioxide analysis for marine studies: Automation and calibration , 1987 .

[11]  D. Wolf-Gladrow,et al.  Calcium carbonate as ikaite crystals in Antarctic sea ice , 2008 .

[12]  A. Gordon,et al.  Winter mixed layer entrainment of Weddell Deep Water , 1984 .

[13]  F. Millero,et al.  A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media , 1987 .

[14]  C. Haas,et al.  Biogeochemical composition of natural sea ice brines from the Weddell Sea during early austral summer , 2007 .

[15]  M. Bender,et al.  Tracers in the Sea , 1984 .

[16]  J. Swift,et al.  Enhanced uptake of atmospheric CO2 during freezing of seawater: A field study in Storfjorden, Svalbard , 2004 .

[17]  A. Körtzinger,et al.  The salinity normalization of marine inorganic carbon chemistry data , 2003 .

[18]  C. Chen,et al.  Why is there little anthropogenic CO2 in the Antarttic bottom water , 1987 .

[19]  S. Doney,et al.  Enhanced CO2 outgassing in the Southern Ocean from a positive phase of the Southern Annular Mode , 2007 .

[20]  M. Hoppema.  Weddell Sea turned from source to sink for atmospheric CO2 between pre-industrial time and present , 2004 .

[21]  A. Lenton,et al.  Role of the Southern Annular Mode (SAM) in Southern Ocean CO2 uptake , 2007 .

[22]  H. J. Zemmelinka,et al.  Stratification and the distribution of phytoplankton , nutrients , inorganic carbon , and sulfur in the surface waters of Weddell Sea leads , 2007 .

[23]  D. Bakker,et al.  The dependence on temperature and salinity of dissolved inorganic carbon in East Atlantic surface waters , 1999 .

[24]  V. Smetácek,et al.  The CD-ROM database of the JGOFS expedition ANT X/6 aboard R.V. Polarstern , 1997 .

[25]  D. Martinson,et al.  The wind-driven circulation in the Weddell-Enderby Basin☆ , 1981 .

[26]  B. Stephens,et al.  The influence of Antarctic sea ice on glacial–interglacial CO 2 variations , 2000, Nature.

[27]  K. Arrigo,et al.  Distributions of Phytoplankton Blooms in the Southern Ocean , 1993, Science.

[28]  P. Christensen,et al.  Inorganic carbon transport during sea ice growth and decay : A carbon pump in polar seas , 2007 .

[29]  H. A. Thomsen,et al.  Nanoplanktonic coccolithophorids (Prymnesiophyceae, Haptophyceae) from the Weddell Sea, Antarctica , 1988 .

[30]  Reiner Schlitzer,et al.  Carbon export fluxes in the Southern Ocean: results from inverse modeling and comparison with satellite based estimates , 2002 .

[31]  Regina Usbeck.  Modeling of marine biogeochemical cycles with an emphasis on vertical particle fluxes (Modellierung mariner biogeochemischer Kreisläufe im Hinblick auf vertikale Partikelflüsse) , 1999 .

[32]  Nicolas Gruber,et al.  The Oceanic Sink for Anthropogenic CO2 , 2004, Science.

[33]  D. Olbers,et al.  Hydrographic Atlas of the Southern Ocean , 1992 .

[34]  Andrew G. Dickson,et al.  Guide to best practices for ocean CO2 measurements , 2007 .

[35]  E. Carmack,et al.  On the flow of water out of the Weddell Sea , 1975 .

[36]  A. Watson,et al.  Southern Ocean iron enrichment promotes inorganic carbon drawdown , 2001 .

[37]  C. Culberson,et al.  MEASUREMENT OF THE APPARENT DISSOCIATION CONSTANTS OF CARBONIC ACID IN SEAWATER AT ATMOSPHERIC PRESSURE1 , 1973 .

[38]  N. Metzl,et al.  An empirical estimate of the Southern Ocean air‐sea CO2 flux , 2007 .

[39]  A. Gordon,et al.  Southern ocean winter mixed layer , 1990 .

[40]  H. Hellmer,et al.  Deep and Bottom Water of the Weddell Sea's Western Rim , 1993, Science.

[41]  Jorge L. Sarmiento,et al.  Redfield ratios of remineralization determined by nutrient data analysis , 1994 .

[42]  L. Anderson,et al.  The transport of anthropogenic carbon dioxide into the Weddell Sea , 1991 .

[43]  R. Weiss,et al.  Surface water and atmospheric carbon dioxide and nitrous oxide observations by shipboard automated gas chromatography: Results from expeditions between 1977 and 1990 , 1992 .

[44]  M. Abbott,et al.  Phytoplankton chlorophyll distibutions and primary production in the Southern Ocean , 2000 .

[45]  A. Gordon,et al.  Weddell Gyre: Temperature maximum stratum , 1989 .

[46]  Gerd Rohardt,et al.  Overflow and Bottom Water Formation In The Southern Weddell Sea , 2004 .

[47]  Wolfgang Roether,et al.  Repeated CFC sections at the Greenwich Meridian in the Weddell Sea , 2002 .

[48]  F. Millero.  Thermodynamics of the carbon dioxide system in the oceans , 1995 .

[49]  A. Winter,et al.  Subtropical coccolithophores in the Weddell Sea , 1999 .

[50]  E. Fahrbach,et al.  New early winter fCO2 data reveal continuous uptake of CO2 by the Weddell Sea , 1999 .

[51]  D. Martinson,et al.  Maud Rise revisited , 2001 .

[52]  T. Whitworth,et al.  Water masses and currents of the Southern Ocean at the Greenwich Meridian , 1987 .

[53]  E. Fahrbach,et al.  Interannual controls on Weddell Sea surface water fCO2 during the autumn–winter transition phase , 2004 .

[54]  E. Fahrbach,et al.  Winter-summer differences of carbon dioxide and oxygen in the Weddell Sea surface layer , 1995 .

[55]  E. Fahrbach,et al.  On the total carbon dioxide and oxygen signature of the circumpolar deep water in the Weddell Gyre , 1997 .

[56]  C. Hanfland,et al.  Actinium-227 as a deep-sea tracer: sources, distribution and applications , 2002 .

[57]  E. Jones,et al.  Oceanic CO2 produced by the precipitation of CaCO3 from brines in sea ice , 1981 .

[58]  J. Plötz,et al.  Winter aggregations of marine mammals and birds in the north-eastern Weddell Sea pack ice , 1991, Polar Biology.

[59]  G. Deacon.  The Weddell gyre , 1979 .

[60]  K. Furuya,et al.  Photosynthetic oxygen production and community respiration in the Indian sector of the Antarctic Ocean during austral summer , 2002, Polar Biology.

[61]  V. Gouretski,et al.  Weddell Gyre structure of the eastern boundary , 1993 .

[62]  C. Tynan.  Ecological importance of the Southern Boundary of the Antarctic Circumpolar Current , 1998, Nature.

[63]  D. Martinson,et al.  A Convective Model for the Weddell Polynya , 1981 .

[64]  Walter H. F. Smith,et al.  Global Sea Floor Topography from Satellite Altimetry and Ship Depth Soundings , 1997 .

[65]  R. Wanninkhof,et al.  Measurement of fugacity of CO2 in surface water using continuous and discrete sampling methods , 1993 .

[66]  M. Hoppema,et al.  CO2 in the Weddell Gyre and Antarctic Circumpolar Current: austral autumn and early winter , 2000 .

[67]  E. Fahrbach,et al.  Annual uptake of atmospheric CO2 by the Weddell Sea derived from a surface layer balance, including estimations of entrainment and new production , 1999 .

[68]  D. Bakker,et al.  Changes of carbon dioxide in surface waters during spring in the Southern Ocean , 1997 .

[69]  M. Hoppema,et al.  Direct measurements reveal insignificant storage of anthropogenic CO2 in the Abyssal Weddell Sea , 2001 .