Date

2019

Department or Program

Geosciences

Primary Wellesley Thesis Advisor

Hilary Palevsky

Additional Advisor(s)

Daniel Brabander

Additional Advisor

Rachel Stanley

Additional Advisor

Erich Matthes

Abstract

The North Atlantic plays a key role in sequestering anthropogenic carbon. The biological carbon pump is one of the mechanisms by which carbon is sequestered in the ocean. Phytoplankton photosynthesize at the sea surface, and fix carbon dioxide into organic carbon, a fraction of which sinks to the deep ocean. For sinking carbon to be stored long-term (time scales > 1 year), it must sink deeper than the winter ventilation depth before being respired. This study focuses on how deep winter ventilation (convective mixing to depths > 1000 m) in the Irminger Sea influences the sequestration of carbon via the biological pump. We analyze a four-year continuous time series (September 2014 to June 2018) of temperature, salinity, dissolved oxygen, optical backscatter, and chlorophyll fluorescence depth profiles from 200 to 2,600 meters from the Apex Profiler Mooring at the Ocean Observatories Initiative (OOI) Global Irminger Sea Array (60.0°N, 39.5°W). Temperature, salinity, and dissolved oxygen data are used to identify the depth and timing of winter mixing. Using dissolved oxygen as a tracer for respiration of sinking organic matter, optical backscatter as a tracer of particulate organic matter, and chlorophyll as an indicator of surface-derived photosynthetic material, the amount of sinking organic carbon ventilated back to the atmosphere in winter, as well as the carbon that escapes below the winter ventilation depth, is evaluated.

We present data collected during a research cruise to the Irminger Sea in June 2018, where we collected discrete samples to calibrate sensors on the OOI Apex Profiler Mooring, and determined variability in the depth of winter mixing across the Irminger basin (59.6˚ – 64.2˚ N and 21.9˚ – 41.4˚ W) using 20 depth profiles of temperature, salinity, and dissolved oxygen from CTD casts. Evidence of deep winter convection was found throughout the basin, though there is variability in the maximum depth of convection, ranging from 1100 to 1500 meters. Using the 2014-2018 time series of data collected by the profiler mooring, we found strong interannual variability in both winter ventilation depths and respiration rates from the in the Irminger Sea, with winter ventilation depths ranging from 800 to 1,150 meters over the four year time period, and depth-integrated respiration rates within the seasonally-ventilated thermocline ranging from 3.62 to 7.66 mol O2 m-2. We find the highest respiration rates in the upper thermocline which contribute the majority of the carbon that is released back into the atmosphere during mixing. We also find that winter ventilation depth influences how much carbon is ventilated into the atmosphere during winter mixing. This study shows the importance of year-round observations in order to better understand how deep convection influences carbon sequestration by the biological pump.

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