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Publication Title | Cooling, dilution and mixing of ocean water by free-drifting icebergs in the Weddell Sea

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Deep-Sea Research II ] (]]]]) ]]]–]]]

Contents lists available at ScienceDirect

Deep-Sea Research II

journal homepage: www.elsevier.com/locate/dsr2

Cooling, dilution and mixing of ocean water by free-drifting icebergs in the Weddell Sea

John J. Helly a,c,n, Ronald S. Kaufmann b, Gordon R. Stephenson Jr.c, Maria Vernet c

a San Diego Supercomputer Center, University of California, San Diego, La Jolla, CA 92093, USA

b Marine Science and Environmental Studies Department, University of San Diego, 5998 Alcala ́ Park, San Diego, CA 92110, USA c Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, USA

article info

Article history:

Received 12 November 2010 Accepted 12 November 2010

Keywords:

Icebergs Antarctica Weddell Sea Carbon cycle Mapping

1. Introduction

The impacts of melting icebergs have been studied previously from a physical oceanographic perspective showing distinctive patterns of temperature and salinity with depth due to meltwater injection into surrounding waters (Donaldson, 1978; Gade, 1979; Huppert, 1980; Jenkins, 1999; Neshyba, 1977). It has also been shown that icebergs can produce biological ‘hot-spots’ that are correlated with the signature of terrestrial inorganic constituents (Smith et al., 2007). Remote sensing data, in combination with in situ tracking of icebergs, have been used to investigate the role of icebergs in mixing surface and sub-surface waters, supporting speculation of a role for icebergs in mixing seawater (Schwarz and Schodlok, 2009). Collectively, these results argue for a direct impact on surface biological productivity and the biogeochemis- try of the carbon cycle through a complex of processes.

In 2005, we detected the influence of icebergs on the physical and biotic environment (Smith et al., 2007) and inferred it to be primarily from surface and near-surface meltwater. This led, in 2008, to the development of the surface mapping approach

n Corresponding author at: San Diego Supercomputer Center, University of California, San Diego, La Jolla, CA 92093, USA. Tel.: +1 858 534 5060;

fax: +1 858 822 3631.

E-mail address: hellyj@ucsd.edu (J.J. Helly).

0967-0645/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2010.11.010

abstract

Iceberg C-18a (35 7 0.184 km) was studied repeatedly by five circumnavigational surveys in March-April 2009. During the period of the surveys, C-18a travelled 109 nautical miles in 23 days covering an area of 8.1 103 km2. This iceberg was formed from iceberg C-18 (76 7 km) that originated from the Ross Ice Shelf in May, 2002. Ship-based measurements show that this iceberg produced fresh meltwater above the seasonal pycnocline that diluted and chilled the water it passed through from the surface to a depth of approximately 50 m (summer mixed layer). The surface meltwater effects were detectable as far away as 19 km and persisted for at least 10 days. We also found evidence that this iceberg was disrupting the Weddell Deep Water to depths up to 1500 m. If we include these deep effects through the water column, the estimate of ocean water altered by this single iceberg reaches 3 1012 m3 over 23 days. Chemical and biological effects were detected at the same space and time scales as the physical properties, with decreasing partial pressure of carbon dioxide (pCO2) close to the iceberg and lower particle and chlorophyll concentration. Ten days after the passage of C-18a, chlorophyll-a had increased by 15%. These results are consistent with alternative hypotheses regarding the role of icebergs as mediators of a localized geophysical disturbance (H1) as well as promoters of chlorophyll-a production (H2).

& 2010 Elsevier Ltd. All rights reserved.

(Helly et al., in press) to characterize the meltwater distribution at the surface. In 2009, we extended our investigation to include the vertical distribution by using expendable bathythermographs (XBTs) and an acoustic-doppler current profiler (ADCP) during the surface mapping surveys. The results from these profiles led to consideration of the mechanical mixing effects of the iceberg as well as the effects of meltwater.

Here we describe the results of our observation of dilution and cooling of the surface waters and disruption of the Weddell Deep Water (WDW) by the meltwater field and wake of a single tabular iceberg, C-18a. These effects were compared with a control site approximately 74 km away and ahead of C-18a away from the area directly affected by C-18a or any other known iceberg (Fig. 1). This area was clear of icebergs for at least 23 days prior to sampling based on microwave satellite imagery. The nature and limitations of this location as a control site are discussed in greater detail below but, in summary, the control location was chosen primarily for geophysical measurements constrained by the expediency of ship operations. More effective control for biological variability will have to be factored into future field research.

The comparison between survey and control sites was done using 2D- and 3D-visualizations from multiple ship-based sensors. These field observations provide evidence for dilution and mixing interactions induced by a combination of mechanical and advective disturbances of the otherwise well-stratified water column in the

Please cite this article as: Helly, J.J., et al., Cooling, dilution and mixing of ocean water by free-drifting icebergs in the Weddell Sea. Deep-Sea Research II (2011), doi:10.1016/j.dsr2.2010.11.010

Image | Cooling, dilution and mixing of ocean water by free-drifting icebergs in the Weddell Sea



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