²³⁴Th‐Derived Particle Fluxes and Seasonal Variability: When Is the SS Assumption Reliable? Insights From a Novel Approach for Carbon Flux Simulation

Abstract

234Th measurements are widely used to estimate the downward carbon flux of particles via the oceanic Biological Pump. Carbon export is evaluated from 234Th‐238U disequilibrium assuming either steady state (SS) conditions, or including a non‐SS (NSS) correction. We use a novel stochastic simulation to quantify the temporal variation of vertical carbon and 234Th (dissolved and particulate) concentration profiles with high temporal resolution. We calculate seasonal export as if in situ measured with sediment trap and SS‐ and NSS‐234Th approaches and quantify the periods of validity for SS/NSS conditions defined in previous works. The SS approach is valid throughout the entire season in oligotrophic regions. In temperate regions, the SS introduces a bias in the export if sampling takes place outside specific temporal windows. Windows of validity range from days in short blooms of ~15‐day duration to weeks in blooms longer than ~30 days.

Sinking Organic Particles in the Ocean—Flux Estimates From in situ Optical Devices

“…These estimates do not, however, provide information about the nature of the particles responsible for the measured fluxes, and rely on assumptions regarding the conversion from radioisotopes to carbon that are difficult to evaluate (Waite and Hill, 2006). Moreover, radioisotopic estimates of particle flux are complicated by potential non-steady state dynamics and physical transport (Ceballos-Romero et al, 2018). Measurements are costly, as they need to be conducted during individual oceanographic cruises, and need to be calibrated with sediment traps.…”

Comment on essd-2021-259

“…The data are archived on PANGAEA repository, with the dataset's DOI https://doi.pangaea.de/10.1594/PANGAEA.918125 (Ceballos-Romero et al, 2021). The data table is available for download either as a unique merged file containing all data sets and metadata or as individual excels files.…”

“…More than 41% of the studies reported POC: 234 Th ratios collected with sediment traps (Haskell et al, 2013;Kawakami et al, 2010;Luo et al, 2014;Maiti et al, 2016). Almost all the studies of the studies reported measurements CHN data (88%), while a very reduced number (~10%) analyzed 210Po (such as Alkalay et al, 2020 and only 3 studies simultaneous measured and discussed 234 Th-derived results from 234 Th, 210Po and compared then to fluxes from sediment traps (Ceballos-Romero et al, 2016;Le Moigne et al, 2013a;and Stewart et al, 2010). Overturning (UK, https://roses.ac.uk/piccolo/).…”

“…In terms of current gaps for application of 234 Th for the assessment of POC fluxes, a fundamental issue is whether we are correctly addressing the temporal conditions at the sampling moment when interpreting 234 Th deficits (Ceballos-Romero et al, 2018). The accuracy of these POC flux evaluations relies on the correct calculation of the 234 Th flux, which is not straightforward given the physical and temporal processes involved in the 234 Th activity balance in a given parcel of water.…”

“…234 Th has been an indispensable tool in many oceanographic field expeditions. The most widespread application of the 234 Th approach is to estimate the gravitational settling of carbon as Particulate Organic Carbon (POC) out of the surface layer, which results in a downward flux of POC (e.g., Bacon et al, 1976;Ceballos-Romero et al, 2016;Kirk Cochran et al, 1993;Rutgers van der Loeff and Geibert, 2008;Stewart et al, 2011;Verdeny et al, 2009). To a lesser extent, this radionuclide is also used to estimate the downward flux of other elements to the deep ocean, such Particulate Inorganic Carbon (PIC) (e.g., Le Moigne et al, (2013a); Wei et al, (2011)), Biogenic Silica (BSi) (e.g., Buesseler et al, (2005); Le Moigne et al, (2013a)), Particulate Organic Nitrogen (PON) (e.g., Buesseler et al, 1992;Charette and Buesseler, 2000;Murray et al, 1996) or trace metals fluxes (e.g., Black et al, 2018;Weinstein and Moran, 2005).…”