Comparative evaluation of sediment trap and ²³⁴Th-derived POC fluxes from the upper oligotrophic waters of the Gulf of Mexico and the subtropical northwestern Pacific Ocean

Abstract

To better understand the inter-relationships between particulate organic carbon (POC) fluxes and phytoplankton and bacteria biomass and production, we compared POC fluxes determined in sediment traps and approaches based on size-fractionated (1–10, 10–50, 50–150 and > 150 μm) suspended particulate ²³⁴Th and POC concentrations in oligotrophic sections of the Gulf of Mexico during August 2005 and May 2006 and in the oligotrophic northwestern Pacific Ocean during 2009. In 2005, the sediment trap POC flux near the bottom of the euphotic zone (120 m) ranged from 71 to 94 mg C m^− 2 day^− 1, while ²³⁴Th-derived POC fluxes using POC/²³⁴Th ratios in the 10–50 µm and 50–150 µm varied from 71 to 150 mg C m^− 2 day^− 1. In 2006, the sediment trap POC flux at 120 m ranged from 24 to 67 mg C m^− 2 day^− 1, while the ²³⁴Th-derived POC fluxes in the 10–50 µm fraction were comparable or somewhat higher, ranging from 71 to 119 mg C m^− 2 day^− 1. The POC fluxes in 2006, calculated by using POC/²³⁴Th ratios in the 1–10 µm and the 50–150 µm fractions were much higher, ranging from 847 to 1369 mg C m^− 2 day^− 1. Correlations with biological and chemical parameters support a likely mechanism of sinking aggregates of haptophytes (0.2–20 µm) of higher density held together by Th-complexing and uronic acid containing exopolymeric substances. The observations that ²³⁴Th (and POC) is mainly associated with medium-sized (10–15 µm) suspended particles rather than larger (50–150 µm) ones may be caused by the use of a one-filter method and standard filtration and processing procedures that were applied here for collecting suspended particles. This then raises the question of what constitutes representative material from the ocean that settles on the characteristic time scale of ²³⁴Th. As a comparison, size-fractionated trap-collected particles in the oligotrophic northwestern Pacific Ocean showed that the 10–50 μm fraction contained the largest proportion of POC (22–41%), followed by the 50–150 μm (22–37%), the > 150 μm (15–27%), and the 1–10 μm (17–23%) fraction. The partitioning of ²³⁴Th in trap-collected particles was slightly different from that of POC, with the 1–10 μm fraction representing the largest proportion (27–48%) of ²³⁴Th flux. Together, the < 50 μm particles contributed, on average, 52 ± 6% of POC, which suggests that the POC/²³⁴Th ratios traditionally derived from large (> 50 µm) pump-collected particles may not accurately reflect the majority of sinking particles. Therefore, estimated POC fluxes may be significantly biased using a conventional ²³⁴Th based approach, i.e., using POC/²³⁴Th ratios from a single filter obtained from large (> 50 µm) pump-collected particles.

Introduction

The “biological pump”, i.e., the removal of CO₂ from the surface ocean via formation and sinking of biological remains, determined as particulate organic carbon (POC), plays a crucial role in the global cycling of carbon, nutrient and particle reactive elements. Therefore, an accurate estimate of POC concentrations and export fluxes from the euphotic zone is crucial for a better understanding of the biogeochemical cycling of carbon in the ocean. Despite possible biases by hydrodynamic and swimmer effects, POC fluxes are often determined by sediment traps (Gardner, 1980, Buesseler, 1991). As an alternative, ²³⁴Th/²³⁸U disequilibrium has been increasingly used for estimating POC fluxes in different marine environments (e.g., Buesseler et al., 1992a, Buesseler et al., 1995, Coale and Bruland, 1985, Cochran et al., 1995, Murray et al., 1996, Buesseler et al., 2006, and references therein). In the latter approach, POC fluxes out of the euphotic zone are determined by the product of the POC/²³⁴Th ratio in sinking particles and the depth-integrated ²³⁴Th flux from the euphotic zone (Buesseler et al., 2006 and references therein). For a reliable POC export flux estimate, both the ²³⁴Th flux and POC/²³⁴Th ratio of sinking particles are required. As for the choice of the appropriate POC/²³⁴Th ratio, it should be representative of the assemblages of particles that sink over a significant vertical distance (at least tens to hundreds of meters) on the characteristic time scale of ²³⁴Th. However, recent studies have shown that ratios of POC/²³⁴Th could vary with water depth, particle size fractions and different hydrographic regimes (e.g., Cochran et al., 1995, Buesseler, 1998, Santschi et al., 1999, Santschi et al., 2003, Moran et al., 2003, Buesseler et al., 2006). While the ratio of POC/²³⁴Th is a crucial question for marine biogeochemistry, our understanding of the mechanisms that control the POC/²³⁴Th ratios is still evolving (Bacon et al., 1996, Murray et al., 1996, Buesseler, 1998, Ducklow et al., 2001, Benitez-Nelson et al., 2001, Coppola et al., 2002, Chen et al., 2003, Santschi et al., 2003, Santschi et al., 2006).

In most studies, filtered large particles (e.g., > 53 or > 70 μm, collected via large volume filtration) are assumed to be representative of the majority of sinking particles (Buesseler et al., 1995, Bacon et al., 1996, Cochran et al., 2000) and the POC/²³⁴Th ratio of such large particles is often adopted to estimate the POC flux in different marine environments (Buesseler et al., 1995, Bacon et al., 1996, Cochran et al., 2000, Benitez-Nelson et al., 2001, Moran et al., 2003, Hung et al., 2004, Buesseler et al., 2006). If this assumption is not universally accepted, we must continue to examine the question: are the filtered large suspended particles really representative of major settling particles in the oligotrophic ocean, where normally pico- and nano-plankton dominate (Campbell et al., 1994, Li, 1994, Moon-van der Staay et al., 2000). Currently, there is no consensus to this question. For example, Coppola et al. (2002) compared POC/²³⁴Th ratios on sinking particles and suspended particles (> 0.6 μm) in the Barents Sea. These authors concluded that the POC flux estimates were more reliable if the POC/²³⁴Th ratios determined from sediment trap material were used. Hung et al. (2004) found that POC fluxes measured by traps and ²³⁴Th derived fluxes (using POC/²³⁴Th on large particles, e.g. > 53 µm, with POC and ²³⁴Th measured separately on different filters) in the Gulf of Mexico were comparable. However, Hung and Gong (2007) reported that the POC export fluxes out of the euphotic zone estimated by sediment traps and ²³⁴Th approaches (using the POC/²³⁴Th ratio on medium-sized particles, e.g., the 10–50 μm) in the Kuroshio Current were in best agreement. Lalande et al., 2008, Lepore et al., 2009 have estimated POC export by using sediment trap and size-fractionated POC/²³⁴Th ratios on suspended particles and found that the estimated POC export by sediment traps and ²³⁴Th/²³⁸U disequilibrium can each be biased, albeit, in different ways. Moreover, Richardson and Jackson (2007) reported that small phytoplankton plays an important role in driving carbon export in the surface ocean. Based on these so far inconsistent results, more field observations are required to investigate which part of the suspended particle spectrum represents sinking particles, and if any discrepancy in results could be produced by the collection method, i.e., filtration. Since sinking particles in the ocean likely settle out as large aggregates, as shown by in situ camera based methods (e.g., Santschi et al., 1999), handling of these aggregates by filtration could produce alterations of the original particle sizes. For this purpose, one needs to compare POC fluxes calculated from POC/²³⁴Th ratios of size-fractionated particles with those from sediment traps to determine which ones are optimal for estimating the POC flux from the euphotic zone.

Surface-tethered floating sediment traps were deployed on the same days as size-fractionated (< 1 µm, 1–10 µm, 10–50 µm, 50–150 µm and > 150 µm) particulate ²³⁴Th and POC samples were collected by large volume in situ pumps from two contrasting (but isolated) oceanographic settings, i.e., cold core rings (CCR) vs. warm core rings (WCR) in the Gulf of Mexico during August 2005 and May 2006. Generally, CCRs contain upwelling nutrient-rich waters with higher primary productivity and higher organic matter fluxes. Conversely, WCRs normally contain more nutrient-depleted waters with lower productivity or biomass, as well as lower organic matter fluxes (Biggs, 1992, Wormuth et al., 2000, Guo et al., 2002a, Santschi et al., 2003). Ring or eddy systems such as these are ideal for addressing our question on ²³⁴Th and POC, as their long-term (∼ 1 year) isolation assures a steady-state system, thus not requiring time-series measurements (Guo et al., 2002a, Santschi et al., 2003, Hung et al., 2004). Given that Station 1 was located in a near-coastal upwelling system and Station 2 was located between a WCR and CCR, the steady-state assumption would not be valid for these stations, and horizontal and vertical transport processes may be important (e.g., Savoye et al., 2006). Our investigation thus allows a comparison between sediment trap-collected POC export fluxes and POC fluxes calculated from an assessment of ²³⁴Th deficiencies combined with POC/²³⁴Th ratios from size-fractionated suspended particles or sediment trap materials. Detailed results of accompanying data sets on particle size distribution of ²³⁴Th, POC, as well as biochemical and biological (i.e., phytoplankton and bacteria) composition and biomass concentration, will be reported elsewhere (Xu et al., in press).

Furthermore, because the ²³⁴Th approach relies on POC/²³⁴Th ratios determined for particles collected by large volume in situ pumps on 50 µm (or similar-sized) mesh, we therefore used a similar sequential filtration system to measure, on the same filter, the abundance of POC and ²³⁴Th concentration in size-fractionated (the 1–10, 10–50, 50–150 and > 150 µm) sinking particles (collected by floating sediment traps) from the oligotrophic waters of the subtropical northwestern Pacific in 2009 to examine what are the representative settling particles and their contribution to the settling flux through the euphotic zone. Besides, we verified the particle size distribution of sinking particles in selected samples from the northwestern Pacific Ocean via scanning electron microscopy (SEM). The POC and ²³⁴Th data from trap-collected particles in the East China Sea and northwestern Pacific Ocean were then compared with the POC and ²³⁴Th data from both pump-collected and trap-collected particles (without size-fractionation) from the Gulf of Mexico.