Zn elemental and isotopic features in sinking particles of the South China Sea: Implications for its sources and sinks

Abstract

We determined the elemental and isotopic composition of Zn in sinking particles collected in the deep water of the northern South China Sea (NSCS) to investigate the relative contribution of various sources and assess their isotopic signatures. Using differentiable elemental ratios and δ⁶⁶Zn of the potential sources, a mass balance approach estimates that anthropogenic aerosol Zn accounted for 64 ± 10% of the total Zn in sinking particles for more than 50% of the sampling period, indicating that anthropogenic aerosol Zn has become a dominant form of Zn source in the deep water. A relatively large discrepancy between the estimated and measured δ⁶⁶Zn is observed during the high productivity season, which can be attributed to the elevated contribution of the biogenic hard parts or scavenging Zn on organic materials. Elevated δ⁶⁶Zn values were observed at 3500 m during autumn which may be caused by the influence of authigenic particles during the lowest flux period. We found that the averaged measured output δ⁶⁶Zn value, +0.35 ± 0.12‰, is significantly lighter than most of the output values proposed in previous studies. Due to recent findings highlighting the importance of anthropogenic aerosol Zn in the ocean, we have re-evaluated the solubility and fluxes of aerosol Zn in the ocean and found that the flux has been significantly underestimated in previous studies. The updated global aerosol Zn input to the ocean, ranging from 0.3 to 3.0 Gmol yr⁻¹, is comparable to the output magnitude from hydrothermal and riverine sources. The updated Zn residence time would then be down to 1400 years on average. In addition to organic decomposition, the sinking particle data indicate that particle-associated removal and release processes play important roles in controlling Zn cycling in the water column. How anthropogenic aerosol deposition influences Zn fluxes and cycling in other oceanic regions deserves further investigation.

Introduction

Anthropogenic activities have considerably increased the input of biologically active elements on Earth and disturbed their natural cycling in the ocean. Among the various transport pathways of anthropogenic material to the ocean, aerosol deposition is considered a major pathway to deliver anthropogenic materials to the ocean (Chester and Jickells, 2012b, Baker et al., 2016). Fossil fuel burning not only introduces CO₂ but also supplies a significant amount of anthropogenic aerosols to the ocean. Anthropogenic aerosol deposition provides a significant amount of reactive nitrogen species to surface waters of the North Pacific Ocean, which may potentially shift nutrient conditions from N-limited to P-limited in the North Pacific Ocean (Kim et al., 2014). Anthropogenic aerosols have also been reported to be an important source of trace metals (e.g., soluble Fe) to surface waters in some oceanic regions, and the impact of anthropogenic aerosol Fe may affect the entire ocean (Chuang et al., 2005, Sholkovitz et al., 2012, Ito et al., 2019). The elevated input of anthropogenic aerosol metals could significantly change trace metal bioavailability or toxicity in seawater for phytoplankton. How anthropogenic aerosol deposition has altered dissolved and particulate metal composition in the euphotic zone and their cycling mechanisms largely remains unknown.

The Northwestern Pacific Ocean (NWPO) and its marginal seas are right next to highly populated East Asia so that the oceanic regions have received tremendous amounts of anthropogenic aerosols during the past few decades (Wang et al., 2015). Two-thirds of the world’s coal combustion was consumed in East Asia, providing a significant amount of anthropogenic aerosol Fe from coal-burning fly ashes to the surface ocean (Lin et al., 2015, Wang et al., 2015; Wang and Ho, 2020). In addition to aerosol Fe input, our studies have demonstrated that anthropogenic aerosol deposition can be a dominant source of many particulate trace metals in surface waters of the NWPO and its marginal seas (Ho et al., 2007, Liao et al., 2017, Liao and Ho, 2018). In the Western Philippine Sea, particulate trace metal to Al and to P ratios were found to be at least one order of magnitude higher than their lithogenic ratios and intracellular quotas, implying that anthropogenic aerosols are a major trace metal source in the surface ocean (Liao et al., 2017). Similarly, previous studies demonstrated comparable patterns in the surface water at the South East Asia Time-series Study (SEATS) station in the northern South China Sea (NSCS, Fig. 1, Ho et al., 2007, Ho et al., 2010). In the deep water of the SEATS station, the elemental composition of sinking particles also suggested that some anthropogenic aerosol metals have been transported down to the deep water with sinking particles (Ho et al., 2011). In this study, the SEATS station was thus chosen to study the contribution of different sources on sinking particle metals in the deep water.

The major Zn inputs to the ocean include fluvial discharge, benthic and hydrothermal inputs, and aerosol deposition (Pacyna and Pacyna, 2001, Chester and Jickells, 2012a). It is worth noting that Zn has been massively used in various human activities, mainly including metallurgy, agriculture, energy production, microelectronics, sewage sludge, and scrap disposal. Among these activities, non-ferrous metal production and fossil fuel combustion account for roughly 85% of total anthropogenic aerosol Zn emissions globally (Pacyna and Pacyna, 2001). Zn is thus highly enriched in anthropogenic aerosols, generally with Zn to Al ratios to be at least two orders of magnitude higher than its crustal ratio (Chester and Jickells, 2012b). Furthermore, anthropogenic aerosol Zn possesses extremely high solubility, and Zn solubility in lithogenic aerosols is relatively low, typically less than 10% (Desboeufs et al., 2005, Shelley et al., 2018). In marine aerosols, previous studies reported that Zn solubility is usually higher than 70% for samples collected either in marginal seas or open oceans (Hsu et al., 2010, Chance et al., 2015, Shelley et al., 2018). High Zn solubility in anthropogenic aerosols indicates that anthropogenic aerosol Zn is the dominant soluble Zn source in the surface ocean (Liao et al., 2020). Although the deposition fluxes of Zn from anthropogenic aerosols may be significant (Nriagu and Pacyna, 1988, Pacyna and Pacyna, 2001, Chester and Jickells, 2012a), its global quantitative contribution still remains unclear.

In addition to elemental ratios and solubility, Zn isotope composition (δ⁶⁶Zn) offers specific information to investigate the sources and processes regulating Zn cycling in the marine water column (Conway and John, 2014, Little et al., 2014, Moynier et al., 2017). Previous studies reported that different Zn sources possess distinguishable δ⁶⁶Zn values, with lithogenic materials to be around +0.3‰ and anthropogenic materials to be generally lighter than lithogenic materials (Cloquet et al., 2006, John et al., 2007b, Chen et al., 2008, Rosca et al., 2019). In terms of seawater, dissolved δ⁶⁶Zn is around +0.47 ± 0.15‰ in deep waters globally, but the surface value is generally isotopically lighter than the deep water in low and mid latitude regions (Bermin et al., 2006, Conway and John, 2014, Conway and John, 2015, John and Conway, 2014, John et al., 2017, Vance et al., 2019, Lemaitre et al., 2020, Liao et al., 2020). Some studies have also reported different fractionation patterns in surface waters, which may be involved with other complicated processes, including external inputs or biological and physiochemical reactions in surface waters (John and Conway, 2014, Vance et al., 2019, Lemaitre et al., 2020, Liao et al., 2020). Traditionally, lithogenic aerosols have been considered as the major source of aerosol Zn in the surface ocean. Recent studies suggested that the surface isotopically light δ⁶⁶Zn values can be attributed to scavenging or an important external Zn input, most likely to be anthropogenic aerosol deposition (Lemaitre et al., 2020, Liao et al., 2020). Zn input originating from anthropogenic aerosol deposition in the marine water column deserves further investigation globally.

In terms of global Zn isotope budget, it is widely accepted that δ⁶⁶Zn in oceanic deep waters is constrained to an average value, +0.47 ± 0.15‰ (Moynier et al., 2017). Riverine, aeolian, hydrothermal, and benthic inputs are considered to be the major sources and Fe-Mn deposits, carbonates, and siliceous sediments are the major sinks (Little et al., 2014, Moynier et al., 2017). The quantitative contribution and representative isotopic values of major inputs and outputs remain uncertain. A couple of studies argued that the δ⁶⁶Zn values of the major inputs to the global ocean are around +0.33‰, which are significantly lighter than the deep water δ⁶⁶Zn value (Little et al., 2014, Vance et al., 2016). These same studies also argued that the values of the major outputs are around +0.90‰, which are heavier than the deep water value. Little et al. (2014) thus proposed that an additional output with isotopically light Zn is needed to balance the budget under steady state assumption. However, based on model simulation, Weber et al. (2018) indicated that the burial Zn in the shelf regions accounts for over 90% of the total Zn output in the global ocean and the δ⁶⁶Zn of the output is +0.36 ± 0.04‰, which is comparable to the value of the major inputs. Thus, the global Zn inputs and outputs can be balanced isotopically. Due to limited field observation and model studies, we believe that it is still at an early stage to confirm representative values for the major inputs and outputs of Zn. It appears that field studies are needed to better assess the isotopic values of the major inputs and outputs in the ocean.

We determined Zn concentrations and isotope composition (δ⁶⁶Zn) in sinking particles collected at 2000 and 3500 m at the SEATS station of the NSCS from April 2014 to March 2015 to evaluate the contribution of different Zn sources to the deep ocean. Using δ⁶⁶Zn and elemental ratios, we have quantified the contribution of anthropogenic aerosol Zn and other possible sources, including sediment resuspension and biogenic materials, in the deep water of the NSCS. To further assess the contribution of anthropogenic aerosol Zn to the global ocean, we have also re-examined all published information about major Zn inputs to the ocean and re-evaluated the importance of aerosol deposition to marine Zn cycling globally.