Global database of oceanic particulate organic carbon to 234 Th ratios: Improving estimates of the biological carbon pump

The ocean’s biological carbon pump (BCP) plays a major role in the global carbon cycle. A fraction of the photosynthetically fixed organic carbon produced in surface waters is exported below the sunlit layer as settling particles (e.g. marine snow). Since the seminal works on the BCP, global estimates of the global strength of the 5 BCP have improved but large uncertainties remain (from 5 to 20 GtC yr -1 exported below the euphotic zone or mixed layer depth). The 234 Th technique is widely used to measure the downward export of particulate organic carbon (POC). This technique has the advantage to allow a downward flux to be determined by integrating the deficit of 234 Th in the upper water column and coupling it the POC/ 234 Th ratio in sinking particles. However, the factors controlling the regional, temporal and depth variations of POC/ 234 Th ratios are poorly understood. We 10 present a database of 9318 measurements of the POC/ 234 Th ratio in the ocean, from the surface down to >5500 m, sampled on three size fractions (~>0.7 µm, ~1-50 µm, ~>50 µm), collected with in situ pumps and bottles, and also from bulk particles collected with sediment traps. The dataset is archived on the data repository PANGAEA® (www.pangaea.de) under https://doi.pangaea.de/10.1594/PANGAEA.911424 (Puigcorbé, 2019). The samples presented in this dataset were collected between 1989 and 2018 and the data have been obtained from published 15 papers and open datasets available online. Unpublished data has also been included. Multiple measurements can be found in most of the open ocean provinces. However, there is an uneven distribution of the data, with some areas highly sampled (e.g, China Sea, Bermuda Atlantic Time Series station) compared to some others that are not well represented, such as the southeastern Atlantic, the south Pacific and the south Indian oceans. Some coastal areas, although in a much smaller number, are also included in this global compilation. Globally, based on 20 different depths horizons and climate zones, the median POC/ 234 Th ratios have a wide range, from 0.6 to 18 µmol dpm -1 .

higher than current concentrations without the biological carbon pump (Parekh et al., 2006). However, quantifying the magnitude of the biological carbon pump at both the regional and global scales is challenging and current assessments vary widely, with estimates ranging from 5 to 20 GtC y -1 being exported below the euphotic zone or the mixed layer depth (Guidi et al., 2015;Henson et al., 2011;Laws et al., 2011).
Here we focus on the use of radioisotopes, specifically, 234 Th. The 234 Th approach allows to quantify an export 45 flux from i) a water profile of 234 Th to obtain its deficit relative to 238 U combined with ii) an estimate of the POC concentration to 234 Th activity (POC/Th ratio) in sinking matter . In reviewing POC/Th ratios variability using the data available at the time,  found that the POC/Th ratios i) increase or remain constant with increasing particle size and ii) decrease with depth. Regionally, the POC/Th ratios vary largely between oceanic provinces and regimes (Puigcorbé et al., 2017a). The study of the 50 biogeochemical behavior of 234 Th with regards to marine particles has received significant attention Le Moigne et al., 2013b;Puigcorbé et al., 2015;Rosengard et al., 2015;Santschi et al., 2006), and the availability of 234 Th related data has been enhanced thanks to international and national programs such as GEOTRACES (Mawji et al., 2015;Schlitzer et al., 2018) JGOFS (Joint Global Ocean Flux Study)  or VERTIGO (Buesseler et al., 2008b), yet the factors controlling the variations of the 55 POC/Th ratio as function of region, time, particle size/type and water column depth remain poorly understood.
Assessing the influence of such factors on the POC/Th ratios will contribute to improve our modelling efforts and our capacity to predict the export and fate of the organic carbon produced in the surface layers. Indeed, the necessity to constrain the variability of the POC/Th was discussed and considered a priority at the technical meeting "The Application of Radionuclides in Studies of the Carbon Cycle and the Impact of Ocean Acidification" 60 held at the International Atomic Energy Agency (IAEA) Environment Laboratories in Monaco in October 2016 (Morris et al., 2017). Therefore, we compiled a database that comprises 9318 POC/Th ratios collected between 1989 to 2018 covering most oceanic provinces at depths ranging from 0 to >5500 m deep. The particles were collected using collection bottles (i.e., Niskin), in situ pumps or sediment traps, and include bulk and size fractionated samples. This database

The crux of the 234 Th approach: POC/Th ratios of sinking particles
The determination of the POC/Th ratio has been historically attained by assuming that sinking carbon is driven by large particles, generally >50 µm in size (researchers also use 51, 53 or 70 µm, depending on the mesh supplier) whereas organic carbon within small particles is assumed to remain suspended and therefore not contribute to the export flux (Bishop et al., 1977;Fowler and Knauer, 1986). However, recent studies have shown that small 100 particles can be significant players in the particle export and should not be disregarded (Alonso-González et al., 2010;Durkin et al., 2015;Le Gland et al., 2019;Puigcorbé et al., 2015;Richardson, 2019), particularly in oligotrophic regions. The most common methods to obtain the particulate fraction to measure the POC/Th ratio are i) in situ pumps (ISP), which can allow for sampling different particle sizes, ii) collection bottles (CB) such as Niskin bottles, providing bulk particles, i.e., >0.7 or 1 µm particles, iii) sediment traps (ST) and although less 105 common iv) marine snow catchers. In some instance various methods have been used in combination (Cai et al., 2010;Maiti et al., 2016;Puigcorbé et al., 2015). Different sampling devices have been shown to provide differences in POC/Th ratios, usually within a factor of 2 to 4 . The differences can be related to the collection of different particles pools and/or the enhanced presence of swimmers. ST collect sinking particles and may suffer from hydrodynamicdiscrimination 110 and undersample slow sinking particles (Gustafsson et al., 2004), while CB sample both, sinking and suspended  (Lepore et al., 2009) and sample neutrally buoyant C-rich aggregates (i.e., nonsinking but with high POC/Th ratios) (Lalande et al., 2008). Biases due to washout of large particles when using ISP (Bishop et al., 2012) or aggregates collapse induced by their high cross-filter pressure (Gardner et al., 2003) may further enhance these differences. The presence of swimmers can also be an important bias of POC/Th ratios 120 when not thoroughly removed, since they skew measurements towards higher values because of their high POC proportion compared to 234 Th Coale, 1990).

POC/ 234 Th ratio variability
Despite the significant body of literature available on POC/Th ratios, more than 10 years after the review by  we still cannot explain the variability of the POC/Th ratios with depth, time, particle type, 125 size or sinking velocity easily nor at a global level. Changes with size and depth have been the most extensively examined. The relation between POC/Th ratio and particle size have been assessed before with results suggesting that there is not a direct relationship. Previous studies have reported increasing ratios with increasing particle size Buesseler et al., 1998;Cochran et al., 2000), which has been interpreted as an effect of the volume to surface area ratio of the particles, due to 234 Th being surface bound whereas C would be contained 130 within the particles ). Yet, a number of studies have reported the opposite trend (i.e., decreasing ratio with increasing particle size; (Bacon et al., 1996;Planchon et al., 2013;Puigcorbé et al., 2015) or no clear change with size Lepore et al., 2009;Speicher et al., 2006). Depth is another factor that has been considered when assessing the variability of POC/Th, since particles are produced in the surface layer and are remineralized on their transit along the water column (Martin et al., 135 1987). POC/Th ratios have been found to be attenuated with depth (Jacquet et al., 2011;Planchon et al., 2015;Puigcorbé et al., 2015). This is due to (in no order or importance) decreasing autotrophic production with increasing water depth, preferential C loss compared to 234 Th through remineralization processes, changes in superficial binding ligands along the water column, and/or scavenging of 234 Th during particle sinking resulting in enhanced particulate 234 Th activities Rutgers van der Loeff et al., 2002); leading to 140 significant variability in the attenuation rates. Theoretically, high sinking velocities may limit the variations of POC/Th ratios with depth, owing to shorter residence times limiting the impacts of biotic and abiotic processes.
However, using specifically design ST that segregate particles according to their in situ sinking velocities, (Szlosek et al., 2009) observed no consistent trend between POC/Th ratios and sinking velocities.
The truth is that numerous processes can impact the POC/Th ratios apart from particle size or depth, such as 145 particle composition or aggregation/disaggregation processes mediated by physical or biological activity (Buesseler and Boyd, 2009;Burd et al., 2010;Maiti et al., 2010;Szlosek et al., 2009) which adds a level of complexity to the prediction of their variability in the ocean. Yet, due to the significance of the POC/Th ratios for the accuracy of the 234 Th flux method, the effort should be made to constrain the factors that will impact its variability and a number of environmental and biogeochemical parameters can be assessed with that goal at a 150 global scale. Among others, surface productivity, phytoplankton composition, zooplankton abundance, mixed layer depth, dust inputs to the surface ocean and ice cover Mahowald et al., 2009;Moriarty and O'Brien, 2013)  We refer to "bulk" (BU) for particles sampled using CB and ISP with a pore size filter of 0.2-1 µm. For this group of samples, particles >0.7 µm were collected using GFF filters and >1 µm using QMA filters. In some particular 165 cases other types of filters, with a different pore size (e.g., 0.2 µm, 0.45 µm or 0.6 µm) might have been used (see database for details). Hereafter, we use >1 µm for the bulk particles. We refer to "small particles" (SP) for particles usually collected using ISP on a 1-50 µm mesh size and "large particles" (LP) for particles usually collected using ISP on mesh size >50 µm (see details on other size ranges also used in the database). Finally, some POC/Th ratios were measured in sinking particles sampled using sediment traps (ST). Figure 1 shows the global distribution of 170 POC/Th ratios grouped by these four categories: BU, LP, SP and ST. The POC/Th ratios were obtained from particles collected at various depths from surface to >5500 m deep ( Figure 2). All the information on locations, dates, depth, size fractions/device (BU, SP, LP and ST) and references are included as metadata in the online database and presented in Table 1.
Our database covers POC/Th measurements sampled between 1989 and 2018, including unpublished data from 175 our laboratories or graciously made available to us by colleagues and data available in online databases. Figure 3 shows the number of POC/Th measurements available per year. In years 1997In years , 2004In years , 2005In years , 2008In years , 2010In years , 2011In years and 2013, the number of POC/Th measurements was >500. This highlights dedicated carbon export programs such as the Joint Global Ocean Flux Study (JGOFS) Murray et al., 1996Murray et al., , 2005, the VERTIGO (Vertical Transport in the Global Ocean) voyages in the Pacific Ocean (Buesseler et al., 2008b), 180 and the GEOTRACES program (Mawji et al., 2015;Schlitzer et al., 2018), as well as the maintained effort of the Time Series Stations (Kawakami et al., 2004(Kawakami et al., , 2010(Kawakami et al., , 2015Kawakami and Honda, 2007). Sampling effort also varied depending on the month of the year (Fig. 3B), with late spring-summer months being the most highly sampled in both hemispheres. The northern hemisphere has been largely sampled in September, May and June (49, 10 and 5 times more data than in the southern hemisphere, respectively), whereas the southern hemisphere 185 has been more sampled in December and February (5 and 4 times more data than the northern hemisphere, respectively), with no data available for the months of July and August and only 5 data points in September (austral winter). For the rest of the months, the northern hemisphere presents 1.4-1.8 times more data than the southern hemisphere. In the equatorial region (taken as the latitudes between -10° and 10° N) major sampling efforts took place in May, with no data collected in January and just 8 data points available from December. The 190 monthly distribution is, therefore, globally biased towards the warmer and more productive seasons, leaving the winter months largely undersampled, particularly in the southern hemisphere.

200
Significant differences were also found between climatic zones, except between the Temperate and Subtropical zones and between the Subtropical and the Tropical zones, when considering all the data together. Statistical differences between zones within a certain depth range are shown in Figure 4.
In general, we observe a reduction in POC/Th ratios with depth, previously reported by others , and likely mainly due to the remineralization of carbon along the water column. The decrease is particularly 205 marked in the upper 200 m, where biological processes affecting the ratios are more intense, and then it smoothes below that depth horizon as the strength of these processes is more limited below the euphotic zone. It is worth noticing that some studies, particularly in coastal areas, presented extremely large POC/Th ratios (> 100 µmol dpm -1 , not included in Figure 4). These high ratios are not always discussed in the publications, but the presence of live zooplankton Savoye et al., 2008;Trull et al., 2008)   . Exceptions do exist, but they are usually found in coastal areas where other 220 factors could be influencing the planktonic community (e.g., seasonal upwelling, continental influence, river inputs).

Contributing to global POC export estimates
The 234 Th approach has been used to derive an export model at global scale that uses sea surface temperatures and net primary productivity from satellite products (Henson et al., 2011). The parametrization for this model has 225 large uncertainties in the cold regions (low sea surface temperature), which lead to a reduced estimate of the global biological carbon pump (~5 GtC y -1 ) compared to other satellite-derived export models (9-13 GtC y -1 ; (Dunne et al., 2007;Laws et al., 2011). A recent study by (Puigcorbé et al., 2017a) estimated POC export fluxes in the North Atlantic using in situ data for the 234 Th method and compared it to three different satellite-derived export models: Dunne et al., 2007;Henson et al., 2011 andLaws et al., 2011. The conclusion was that, overall, the geographical 230 trends were captured by all the approaches, but the absolute values between them could reach important 235 discrepancies. In that study, the authors advised for a revision of the parametrization of the models going beyond sea surface temperatures in order to adjust to specific ocean bioregions. This database sets a strong background to develop that parametrization and contribute to similar modeling efforts to constrain the global carbon export fluxes as done by Henson et al., (2011). It would be beneficial for future efforts to obtain data for those undersampled areas with high seasonality to better characterize the expected variability in the ratios within those areas and to cover a larger span of seasons in order 250 to better understand the seasonality of POC/Th and thus be able to translate it more accurately to the global POC export estimates.