Mesoscale biogeochemical responses to iron fertilization in the upper layers of the Southern Ocean Iron Experiment areas

Abstract

two iron fertilization experiments were conducted at the south (66.45°S, 171.8°W) and north (56.23°S, 172°W) patches. The south patch was replete with all macronutrients, whereas the north patch was nearly depleted of silicate. Using a towed water sampling/ measurement system, high resolution three-dimensional observations of temperature, salinity, CO 2 partial pressure (pCO 2 ) and the concentrations of total CO 2 (TCO 2 ), nitrate, phosphate and silicate in the upper 100 m were obtained 10 and 28 days after initial iron additions in the south and north patches, respectively. TCO 2 and nutrient drawdowns observed in the south patch were small due to wind events that limited biological response in the early postfertilization period. At the north patch, drawdown signals were stronger, resulting from the longer time since fertilization. The north patch was dispersed along a frontal zone between two distinct waters, with the denser subducted under the lighter. Subduction limited CO 2 uptake from the air, but provided alternative mechanisms for vertical carbon export. Based on the biogeochemical drawdowns and published estimates of the rate of the entrainment of nonfertilized waters into the patch, the mean net community productivity over 28 days was estimated to be 94 AE 12 mmol carbon m À2 day À1, to which the air-to-sea flux contributed only 2%. Despite low silicate concentrations and potential limitation of growth of silicifying plankton in the north patch, our estimates of the enhanced carbon uptake are similar to estimates from higher silicate environments.

Two decades of inorganic carbon dynamics along the Western Antarctic Peninsula

“…Short-term exposure to low levels of arag may cause severe dissolution of live pteropod shells and has already been observed in the Scotia Sea (Bednaršek et al, 2012). Surface aragonite undersaturation along the WAP may be a result of ocean acidification and may not have been common during preindustrial times (Hauri et al, 2015).…”

A regression modeling approach for studying carbonate system variability in the northern Gulf of Alaska

“…This resulted in a distinct subsurface minimum in p CO 2 derived from our MLR relationships (Figure ). Subsurface p CO 2 minima have been previously observed only in data collected from towed platforms [ Bandstra et al ., ; Hales and Takahashi , ], and may be under‐appreciated because of the difficulty in resolving these features using discrete bottle measurements. Figure shows a single depth profile from the GLOBEC SeaSoar survey (Figure ) that illustrates the processes driving the existence of subsurface p CO 2 minima in this region.…”

“…Autonomous direct carbonate system measurements are limited to either fixed or navigable surface platforms [ Cullison Gray et al ., ; Evans et al ., ; Orton et al ., ; Willcox et al ., ; Harris et al, submitted manuscript, 2012; Evans and Mathis, Sea‐air CO 2 exchange in the Gulf of Alaska coastal ocean, manuscript in preparation, 2013, and Mathis et al, Constraining the physical and biological controls on carbonate mineral saturation states in the Northern Gulf of Alaska, manuscript in preparation, 2013, hereinafter referred to as Mathis et al, manuscript in preparation, 2013]. To date, only one group has combined ship‐based direct carbonate system measurements with a pumping system mounted on a towed platform capable of highly resolved depth profiles [ Bandstra et al ., ; Hales et al ., ; Hales and Takahashi , ; ; Hales et al ., ]. This system is extremely powerful at resolving fine spatial scale carbonate system structure in the water column, but is still limited by its use on ships.…”

Two decades of inorganic carbon dynamics along the West Antarctic Peninsula

“…Trend analysis (1999Trend analysis ( -2013 of Lamont-Doherty Earth Observatory of Columbia University (LDEO) surface continuous underway pCO 2 (µatm), salinity, and temperature ( • C) measurements from within the central subregion of the Palmer-Long Term Ecological Research (PAL-LTER) sampling grid (Fig. 1, red in the mixed layer south of the polar front, and an even lower N / P ratio of 11.3 ± 0.3 was observed in the ironspiked mixed layer during the iron fertilization experiment in the Subantarctic South Pacific (Hales and Takahashi, 2012). Consistent with the low N / P ratio, the observed C : P ratio (80.5 ± 2.5, corrected for gas exchange) was also lower than the classic Redfield ratio.…”