Abstract
In order to determine the dynamics of marine fluorescent organic matter (FOM) using high-resolution spatial data, in situ fluorometers have been used in the open ocean. In this study, we measured FOM during the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) expedition from early December 2019 to early February 2020, using an in situ fluorometer at 148 stations along the two meridional transects (at ∼80 and ∼57°E) in the Indian Ocean, covering latitudinal ranges from ∼6°N to ∼20°S and ∼30 to ∼65°S, respectively. The FOM data obtained from the fluorometer were corrected for known temperature dependence and calibrated using FOM data measured onboard by a benchtop fluorometer. Using the relative water mass proportions estimated from water mass analyses, we determined the intrinsic values of FOM and apparent oxygen utilization (AOU) for each of the 12 water masses observed. We then estimated the basin-scale relationship between the intrinsic FOM and the AOU, as well as the turnover time for FOM in the Indian Ocean (410 ± 19 years) in combination with the microbial respiration rate in the dark ocean (>200 m). Consistent to previous estimates in the global tropical and subtropical ocean, the FOM turnover time obtained is of the same order of magnitude as the circulation age of the Indian Ocean, indicating that the FOM is refractory and is a sink for reduced carbon in the dark ocean. A decoupling of FOM and AOU from the basin-scale relationship was also observed in the abyssal waters of the northern Indian Ocean. The local variability may be explained by the effect of sinking organic matter altered by denitrification through the oxygen-deficient zone on enhanced abyssal FOM production relative to oxygen consumption.
Highlights
The carbon inventory of marine dissolved organic matter (DOM) is estimated to be approximately 662 Pg C, which is the second largest among the bioreactive carbon reservoirs in the ocean (Hansell et al, 2009)
This study aims to: (1) establish in situ fluorometer application in the open ocean, (2) clarify the characteristic fluorescent DOM (FDOM) values for each of the water masses found along the meridional section in the Indian Ocean to estimate its turnover time and understand its behavior, and (3) provide spatially high-resolution FDOM data that can be directly compared with data collected in other oceanic locations in the past or future
From the relative proportions of the water masses estimated by the optimum multiparameter analysis (OMPA) analyses, we estimated the archetypal FOM370/440 and apparent oxygen utilization (AOU) values for each water mass that was considered in the analyses
Summary
The carbon inventory of marine dissolved organic matter (DOM) is estimated to be approximately 662 Pg C, which is the second largest among the bioreactive carbon reservoirs in the ocean (Hansell et al, 2009). Jørgensen et al (2011) clarified that humic-like fluorophores had positive correlations with AOU by using the global distributions of FDOM determined with excitation-emission matrix (EEM) fluorescence and parallel factor analyses (PARAFAC). The first three studies demonstrated that FDOM/CDOM are produced in the ocean interior via microbial respiration, based on positive correlations between FDOM and AOU or between CDOM and AOU These studies analyzed FDOM/CDOM and AOU data on a basin or global scale by combining them, but without considering the differences in the specific values of each parameter between the water masses. This makes it difficult to obtain a characteristic basin scale or global scale relationship between FDOM and AOU, or between CDOM and AOU. In the Indian Ocean, their transect survey was conducted only in the subtropical region, which indicates that the basin-scale information on FDOM/CDOM is insufficient
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