Abstract
Two Argo floats equipped with oxygen, chlorophyll (Chl), backscatter, and nitrate sensors conducted daily vertical profiles of the water column from a depth of 2000 m to the sea surface in the western North Pacific from January to April of 2018. Data for calibrating each sensor were obtained via shipboard sampling that occurred when the floats were deployed and recovered. Float backscatter observations were converted to particulate organic carbon (POC) concentrations using an empirical relationship derived from contemporaneous float profiles of backscatter and shipboard observations of suspended organic carbon particles. During the float deployment periods, repeated meteorological disturbances (storms) passed over the study area and caused the mixed layer to deepen. During these events, nitrate was entrained from deeper layers into the surface mixed layer, while Chl and POC in the surface mixed layer were redistributed into deeper layers. After the storms, the upper layer gradually restratified, nitrate concentrations in the surface layer decreased, and Chl and POC concentrations increased. When the floats observed the same water mass, the net community production within the euphotic layer (0–70 m), determined from the increases in POC, was 126–664 mg C m−2 d−1 (10.5–55.3 mmol C m−2 d−1) close to the values reported from a nearby area. The C/N ratio of the increase in POC and the decrease in nitrate was closed to the Redfield ratio, which indicates that the sensors were able to observe the net biochemical processes in this area despite the relatively low concentrations of nitrate and POC. To determine the fate of particles transported from the surface ocean to the twilight layer, the ratio of oxygen consumption and nitrate regeneration rates were compared. This O2/N ratio approached the Redfield ratio when the floats followed the same water mass continuously, but the consumption rate of POC was significantly lower than what would be expected based on the oxygen consumption and nitrate release rates. This suggests that dissolved organic carbon was the main substrate for the respiration in the twilight layer.
Highlights
Particulate matter in the ocean plays an important role in the global carbon cycle
Vertical bbp profiles showed spike-like signals in some places, and these signals may suggest the presence of large particles (Bishop and Woods, 2008; Briggs et al, 2011)
The particulate organic carbon (POC) profiles showed no spiked values, which may indicate that the large particles that cause spiked signals were only present to the extent that they did not affect the amount of POC
Summary
Particulate matter in the ocean plays an important role in the global carbon cycle. Sinking organic particles transport carbon from the surface to the ocean interior, sequester carbon in the deep ocean, and act to lower the atmospheric CO2 concentration (Sarmiento and Gruber, 2006; Giering et al, 2020). The vertical transport of particulate organic carbon (POC) has mainly been ascribed to the gravitational sinking of particles (Sanders et al, 2014; Seigel et al, 2016; Boyd, et al, 2019; Resplandy, et al, 2019). The crucial factor for the magnitude and efficiency of the biological pump is the sinking velocity of particles. Fast-sinking particles (i.e., > 20 m d-1) could efficiently transport POC to the ocean interior with little remineralization over a short period of time (Billett et al, 1983). Slow-sinking and suspended particles are likely remineralized at shallow depths, returning CO2 to the water column and limiting the efficiency of the biological pump (Buesseler et al, 2007). One key reason for this is the difficulty associated with observing the behavior of sinking or suspended particles in the water column. It is hoped that the growing use of these sensors on Argo floats will make it possible to acquire data during any season and supply enough information to study particle transport in more detail
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