Abstract
Particulate inorganic carbon (PIC) plays a major role in the ocean carbon cycle impacting pH, dissolved inorganic carbon, and alkalinity, as well as particulate organic carbon (POC) export and transfer efficiency to the deep sea. Remote sensing retrievals of PIC in surface waters span two decades, yet knowledge of PIC concentration variability in the water column is temporally and spatially limited due to a reliance on ship sampling. To overcome the space–time gap in observations, we have developed optical sensors for PIC concentration and flux that exploit the high mineral birefringence of CaCO3 minerals, and thus enable real-time data when deployed operationally from ship CTDs and ARGO-style Carbon Flux Explorer floats. For PIC concentrations, we describe a fast (10 Hz) digital low-power (∼0.5 W) sensor that utilizes cross-polarized transmitted light to detect the photon yield from suspended birefringent particles in the water column. This sensor has been CTD-deployed to depths as great as 6,000 m and cross-calibrated against particulates sampled by large volume in situ filtration and CTD/rosettes. We report data from the September–November 2018 GEOTRACES GP15 meridional transect from the Aleutian Islands to Tahiti along 152°W where we validated two prototype sensors deployed on separate CTD systems surface to bottom at 39 stations, many of which were taken in nearly particle-free waters. We compare sensor results with major particle phase composition (particularly PIC and particulate aluminum) from simultaneously collected size-fractionated particulate samples collected by large volume in situ filtration. We also report results from the June 2017 California Current Ecosystem-Long Term Ecological Research (CCE-LTER) process study in California coastal waters where high PIC levels were found. We demonstrate that the PIC concentration sensor can detect PIC concentration variability from 0.01 to >1 μM in the water column (except in nepheloid layers) and outline engineering needs and progress on its integration with the Carbon Flux Explorer, an autonomous float.
Highlights
Phytoplankton account for half of global net photosynthesis (Field et al, 1998; Falkowski et al, 1988; Antoine et al, 1996), or about 50 Pg C y−1, yet they live for a week before being removed from the euphotic zone
The GEOTRACES GP15 section is notable as it transited waters of the Aleutian shelf, Alaska gyre, subarctic front, north Pacific gyre, the complex current regime of the equatorial Pacific, and into the south Pacific Gyre
The core of the westward flowing North Equatorial Current (NEC) which completes the circulation of the north Pacific gyre is found at ~14°N; the front demarking the North Equatorial Counter Current (NECC) is denoted by strong upward displacement of isotherms and oxygen-depleted waters extending from 100 to 350 m at 11°N
Summary
Phytoplankton account for half of global net photosynthesis (Field et al, 1998; Falkowski et al, 1988; Antoine et al, 1996), or about 50 Pg C y−1, yet they live for a week before being removed from the euphotic zone. Losses of particulate organic carbon (POC) from the euphotic zone to deeper waters sum to an uncertain 5–12 Pg C yr−1 (Boyd and Trull, 2007; Henson et al, 2011; Li and Cassar, 2016; Dunne et al, 2005; Siegel et al, 2014, 2016; Yao and Schlitzer, 2013) and constitute the ocean biological carbon pump (OBCP, Volk and Hoffert, 1985). This process of photosynthetic transformation of dissolved inorganic carbon to organic matter, and its export, leads to an increase of pH and a decrease of the CO2 concentration in surface waters, enhancing the transfer of CO2 from the atmosphere to the ocean. It is not presently possible to determine from ship observations if and how the biological carbon pump may be changing in response to anthropogenic ocean acidification (Feely et al, 2004; Sabine et al, 2004; Orr et al, 2005), nor is it possible to gauge the effects of warming-induced changes in stratification and circulation without higher frequency observations
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