Dennis A. Hansell is a professor in the Department of Ocean Sciences at the University of Miami. His research interests are in the biogeochemistry of the major elements (such as carbon and nitrogen), primarily conducted by inferring the biological processing of those elements from their spatial and temporal variations.
Hansell teaches courses in ocean biogeochemistry at both the undergraduate and graduate levels, and mentors graduate students and postdoctoral scientists toward careers as professional scientists.
The contributions of total organic carbon and nitrogen to elemental cycling in the surface layer ... more The contributions of total organic carbon and nitrogen to elemental cycling in the surface layer of the Sargasso Sea are evaluated using a 5-yr time-series data set (1994-1998). Surface-layer total organic carbon (TOC) and total organic nitrogen (TON) concentrations ranged from 60 to 70 µM C and 4 to 5.5 µM N seasonally, resulting in a mean C : N molar ratio of 14.4±2.2. The highest surface concentrations varied little during individual summer periods, indicating that net TOC production ceased during the highly oligotrophic summer season. Winter overturn and mixing of the water column were both the cause of concentration reductions and the trigger for net TOC production each year following nutrient entrainment and subsequent new production. The net production of TOC varied with the maximum in the winter mixed-layer depth (MLD), with greater mixing supporting the greatest net production of TOC. In winter 1995, the TOC stock increased by 1.4 mol C/m**2 in response to maximum mixing depths of 260 m. In subsequent years experiencing shallower maxima in MLD (<220 m), TOC stocks increased <0.7 mol C/m**2. Overturn of the water column served to export TOC to depth (>100 m), with the amount exported dependent on the depth of mixing (total export ranged from 0.4 to 1.4 mol C/m**2/yr). The exported TOC was comprised both of material resident in the surface layer during late summer (resident TOC) and material newly produced during the spring bloom period (fresh TOC). Export of resident TOC ranged from 0.5 to 0.8 mol C/m**2/yr, covarying with the maximum winter MLD. Export of fresh TOC varied from nil to 0.8 mol C/m**2/yr. Fresh TOC was exported only after a threshold maximum winter MLD of ~200 m was reached. In years with shallower mixing, fresh TOC export and net TOC production in the surface layer were greatly reduced. The decay rates of the exported TOC also covaried with maximum MLD. The year with deepest mixing resulted in the highest export and the highest decay rate (0.003 1/d) while shallow and low export r [...]
Eos, Transactions American Geophysical Union, Jul 28, 2015
Controversy leads to a better understanding of carbon cycling through a massive pool of organic m... more Controversy leads to a better understanding of carbon cycling through a massive pool of organic matter dissolved in the Earth's oceans.
ABSTRACT During the recent GasEx-98 cruise in the North Atlantic aboard the NOAA ship Ronald H. B... more ABSTRACT During the recent GasEx-98 cruise in the North Atlantic aboard the NOAA ship Ronald H. Brown, carbon measurements were performed in the area of 46°N, 20.5°W. This process study followed a warm core ring tagged with the deliberately introduced tracer, SF6. Continuous surface water measurements were combined with vertical profiles sampled daily to depths up to 1000 m for carbon mass balance studies. Dissolved inorganic carbon (DIC) and fCO2 measurements were conducted onboard in both underway and discrete analysis modes. During the 25-day experiment in the tagged patch surface water fCO2 values averaged 275±9 μatm, providing a constant condition of undersaturation and flux of CO2 into the ocean. Using the Wanninkhof [1992] exchange coefficient, the estimated CO2 flux ranged from approximately 1-27 mol m-2 yr-1 . The largest CO2 flux occurred during a large wind event beginning on June 6. After the event, DIC and fCO2 values decreased for a few days, as a result of increased productivity associated with the strong mixing event. The DIC results were combined with the TOC, TON, and nutrient data to provide a mass balance for carbon within the patch. The results for the 25-day period indicate DIC increases in the mixed layer ranging from 0.2-1.8 μmol kg-1 d-1 due to gas exchange.
... Zamora, Lauren, Cross, Jessica, Hansell, Dennis and Prospero, Joseph (2007) The relationship ... more ... Zamora, Lauren, Cross, Jessica, Hansell, Dennis and Prospero, Joseph (2007) The relationship between source and soluble phosphorus content in aerosols from Miami and Barbados [Poster] In: SOLAS Summer School, 29.08-10.09.2007, Corsica, France. ...
The contributions of total organic carbon and nitrogen to elemental cycling in the surface layer ... more The contributions of total organic carbon and nitrogen to elemental cycling in the surface layer of the Sargasso Sea are evaluated using a 5-yr time-series data set (1994-1998). Surface-layer total organic carbon (TOC) and total organic nitrogen (TON) concentrations ranged from 60 to 70 µM C and 4 to 5.5 µM N seasonally, resulting in a mean C : N molar ratio of 14.4±2.2. The highest surface concentrations varied little during individual summer periods, indicating that net TOC production ceased during the highly oligotrophic summer season. Winter overturn and mixing of the water column were both the cause of concentration reductions and the trigger for net TOC production each year following nutrient entrainment and subsequent new production. The net production of TOC varied with the maximum in the winter mixed-layer depth (MLD), with greater mixing supporting the greatest net production of TOC. In winter 1995, the TOC stock increased by 1.4 mol C/m**2 in response to maximum mixing depths of 260 m. In subsequent years experiencing shallower maxima in MLD (<220 m), TOC stocks increased <0.7 mol C/m**2. Overturn of the water column served to export TOC to depth (>100 m), with the amount exported dependent on the depth of mixing (total export ranged from 0.4 to 1.4 mol C/m**2/yr). The exported TOC was comprised both of material resident in the surface layer during late summer (resident TOC) and material newly produced during the spring bloom period (fresh TOC). Export of resident TOC ranged from 0.5 to 0.8 mol C/m**2/yr, covarying with the maximum winter MLD. Export of fresh TOC varied from nil to 0.8 mol C/m**2/yr. Fresh TOC was exported only after a threshold maximum winter MLD of ~200 m was reached. In years with shallower mixing, fresh TOC export and net TOC production in the surface layer were greatly reduced. The decay rates of the exported TOC also covaried with maximum MLD. The year with deepest mixing resulted in the highest export and the highest decay rate (0.003 1/d) while shallow and low export r [...]
Eos, Transactions American Geophysical Union, Jul 28, 2015
Controversy leads to a better understanding of carbon cycling through a massive pool of organic m... more Controversy leads to a better understanding of carbon cycling through a massive pool of organic matter dissolved in the Earth's oceans.
ABSTRACT During the recent GasEx-98 cruise in the North Atlantic aboard the NOAA ship Ronald H. B... more ABSTRACT During the recent GasEx-98 cruise in the North Atlantic aboard the NOAA ship Ronald H. Brown, carbon measurements were performed in the area of 46°N, 20.5°W. This process study followed a warm core ring tagged with the deliberately introduced tracer, SF6. Continuous surface water measurements were combined with vertical profiles sampled daily to depths up to 1000 m for carbon mass balance studies. Dissolved inorganic carbon (DIC) and fCO2 measurements were conducted onboard in both underway and discrete analysis modes. During the 25-day experiment in the tagged patch surface water fCO2 values averaged 275±9 μatm, providing a constant condition of undersaturation and flux of CO2 into the ocean. Using the Wanninkhof [1992] exchange coefficient, the estimated CO2 flux ranged from approximately 1-27 mol m-2 yr-1 . The largest CO2 flux occurred during a large wind event beginning on June 6. After the event, DIC and fCO2 values decreased for a few days, as a result of increased productivity associated with the strong mixing event. The DIC results were combined with the TOC, TON, and nutrient data to provide a mass balance for carbon within the patch. The results for the 25-day period indicate DIC increases in the mixed layer ranging from 0.2-1.8 μmol kg-1 d-1 due to gas exchange.
... Zamora, Lauren, Cross, Jessica, Hansell, Dennis and Prospero, Joseph (2007) The relationship ... more ... Zamora, Lauren, Cross, Jessica, Hansell, Dennis and Prospero, Joseph (2007) The relationship between source and soluble phosphorus content in aerosols from Miami and Barbados [Poster] In: SOLAS Summer School, 29.08-10.09.2007, Corsica, France. ...
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