Biases in the in situ measurement of particulate organic carbon and its effect on the calibration and validation of ocean color sensors.

Joaquin Ernesto Chaves, NASA Goddard Space Flight Center, Easton, MD, United States, Michael Novak, NASA Goddard Space Flight Center, Greenbelt, MD, United States, Ivona Cetinic, NASA Goddard Space Flight Cent, Greenbelt, United States and Antonio Mannino, NASA Goddard Space Flight Center, Ocean Ecology Laboratory, Greenbelt, MD, United States
Particulate organic carbon (POC) plays an oversized role, relative to its standing stock, in the global C cycle as a pathway for C sequestration. Accurate measurement of POC is central to understanding the ocean C flux and its sensitivity to climate forcing, and critical to the NASA EXPORTS field campaigns. POC is a standard NASA ocean color data product, which lacks a consensus, quality-assured measurement protocol for satellite validation. Thus, algorithms based on field measurements lacking verified uncertainties have limited applicability towards climate data records (CDRs). Different sampling and filtration protocols, and blank corrections, introduce biases in the magnitude of POC measured from the field. A significant filter blank attributable to dissolved organic C adsorption that, until recently has been seldom corrected for, likely has introduced biases in POC global datasets. The relative magnitude of this error is potentially higher in oligotrophic regions, which due to their areal extent, have a large effect on model and remote sensing estimates of global C reservoirs and fluxes.

As part of an ongoing effort to develop standardized protocols, we measured near-surface POC during the 2017 P06 GO-SHIP campaign across the South Pacific Gyre and into the Chilean Upwelling. In situ POC corrected for filter blank was almost always lower than ocean color derived values--satellite values had a ~43% bias above in situ. The implication, if proven generalizable, is that current POC algorithms based on non-corrected data might be overestimating this parameter. Furthermore, differences in the extent of the POC overestimate across ocean ecosystem trophic condition has serious ramifications for interpreting past and current measurements applied to derive C export. Results from this investigation will help improve estimates for POC from ocean color and their assimilation into models and CDRs.