Fluxes and isotopic composition of planktonic foraminifera off Hainan Island, northern South China Sea: Implications for paleoceanographic studies

Abstract

Planktonic foraminifera collected from a sediment trap deployed off Hainan in the northwestern South China Sea (SCS-NW) between July 2012 and April 2013 were studied to evaluate their seasonal variability and ecology as well as to infer the factors controlling their shell fluxes. The total planktonic foraminifera flux, as well as the fluxes of the dominant species (Globigerinoides ruber, Globigerinoides sacculifer and Neogloboquadrina dutertrei), showed three distinct maxima during SW-monsoon in August 2012, the SW-NE intermonsoon in October 2012 and the NE-monsoon in December 2012–February 2013. These periods were characterized by upwelling, aerosol fallout, and intense wind mixing, respectively, from which the foraminiferal assemblages benefitted, as indicated by the close correlation between wind speed, sea surface temperature (SST), chlorophyll a concentration (Chl-a), δ¹⁸O of G. ruber and the shell fluxes. The correlation also suggests that temperature and food availability might have been the primary drivers of the observed changes in foraminiferal abundance. The offset between the SST deduced from flux-weighted of G. ruber δ¹⁸O and annual mean SST is only ∼0.3 °C, much lower than ∼5.2 °C between the summer and winter temperature, indicating a balanced seasonality bias in the shell flux. The linear regression between the satellite-derived sea surface temperature and G. ruber δ¹⁸O reveals the strong potential of this species, at least in the studied region, as an ecological indicator for past oceanic environments.

Introduction

The most commonly used proxies in the study of paleoceanography include the planktonic foraminiferal assemblages, the stable oxygen and carbon isotopes as well as trace-element composition of their shells (Lea, 1999, Rohling and Cooke, 1999), and the composition of their shell-bound organic matter (King and Hare, 1972a, King and Hare, 1972b, Langer et al., 1993, Stathoplos and Tuross, 1994, Uhle et al., 1997). These are applied to generate quantitative estimates of ocean boundary conditions, such as the sea surface temperature (SST), salinity (SSS), and those of biotic response, including primary production and carbon flux.

These proxies from planktonic foraminifera are widely used in paleoceanographic studies, not only because of their assemblages that vary with marine environmental changes but also because of the ability of their shells to preserve useful information about the upper oceans in which they once lived. In the past decades, oceanic parameters have been established by the abundance of planktonic foraminifera, the stable oxygen and carbon isotope compositions of their shells for temperature, upwelling, nutrient enrichment, and productivity. However, these paleoceanographic signals present in the foraminiferal shell could be affected by physiological effects and create both “oxygen” and “carbon” isotope disequilibrium relative to the ambient seawater (Niebler et al., 1999).

Therefore, a good knowledge of ecology, life cycle, and shell calcification processes of the individual foraminiferal species is required for better interpretation of the fossil record (Niebler et al., 1999). For instance, Globigerinoides ruber is a mixed layer subtropical and tropical species, and a good recorder of near-surface temperature because of its low-slope response to a deepening isotherm (high insulation for the low temperature at depth compared to other species). This makes it a good species for surface water temperature reconstruction (Field, 2004, Tedesco et al., 2007, Wejnert et al., 2010).

Seasonal variation in the shell flux in the three seasonality modes described by Jonkers and Kučera (2015) could also serve as useful tools in the understanding and interpretation of shell-flux variability pattern, and their applicability in paleoceanographic reconstruction. For example, seasonal variation in shell flux in the warm water planktonic foraminiferal species (group A) was reported to be low within the optimal temperature range. In that case, species record average annual condition with only little offset, but outside their optimal temperature range, species concentrate a progressively large portion of their annual flux into a single peak in autumn. This often results in an increase in positive bias from mean annual conditions. Under both the cooling and warming trends, the species seasonality shifts toward an odd flux throughout the year, which will lead to underestimation of the actual changes in mean temperature (Jonkers and Kučera, 2015).

Recent publications on environmental investigations of the South China Sea (SCS) on the basis of planktonic foraminifera were largely focused on their seasonal and interannual variations in the central SCS (Chen et al., 2000, Chen et al., 2007), seasonal variability off Xisha Island (Liu et al., 2014, Xiang et al., 2015), variation in their flux and chemical properties in the southern basin (Wan et al., 2010), assemblage composition in plankton tows from the northern SCS (Lin and Hsieh, 2007), circulation and oxygenation of the glacial SCS (Li et al., 2017), in situ calibration of the Mg/Ca ratio in their shells (Huang et al., 2008), upper water-column structure and non-analog planktonic foraminiferal fauna in the glacial southern SCS (Steinke et al., 2008, Steinke et al., 2010).

The SCS is a highly dynamic monsoonal subregion characterized by a variety of processes that govern the fluxes of particulate matter to the deep sea. This includes massive riverine discharges of sediments and nutrients, upwelling, mesoscale eddies, dust fallout, and El Ninṏ-southern oscillation (ENSO) events (see Section 1.1). Therefore, because of this multiplicity, a quantitative understanding of the individual forcings controlling the distribution of planktonic foraminifera is required. The objective of this study is to improve our understanding of the ecology, seasonal variability, and possible factors mediating the shell fluxes of some observed planktonic foraminifera in the northern SCS.

The South China Sea (SCS), which is the largest marginal sea in the western tropical Pacific Ocean, has an area of about 3.5 million km² and depths ranging from the shallowest coastal region to 5567 m, with an average depth of 1212 m (Morton and Blackmore, 2001). It is located within the East Asian Monsoon system with sea surface temperature (SST) and sea surface salinity (SSS) varying between <24 °C and >29 °C, and 34.2 psu and 33.6 psu, during winter and summer seasons respectively in the north (Fig. 1, Locarnini et al., 2013), except for its coastal areas. It is a typical well-stratified oligotrophic sea with depleted surface nutrients, thereby manifesting low primary production in the surface layer (Wong et al., 2007, Du et al., 2017).

The biogeochemistry of the SCS is responsive to multiscale physical forcings, such as intraseasonal upwelling and mesoscale eddies (Liu et al., 2002, Ning et al., 2004), the seasonally reversing monsoon, and the interannual ENSO events (Liu, H. et al., 2013). The effects may be recorded in changes in nutrient concentrations, Chl-a, primary production, and POC export. The seasonally varying hydrological characteristics of the SCS are complex (Fig. 1a). The East Asian monsoon (northeast monsoon in winter and southwest monsoon in summer) is the primary driver, resulting in a large basin-scale cyclonic upper layer circulation during winter and sub-basin-scale cyclonic and anticyclonic gyres during summer (Fig. 1a; Wyrtki, 1961).

Mixed layer depth (MLD), which is modulated by wind stress and solar radiation, exhibits pronounced seasonal variability in the northern SCS (Tseng et al., 2005). Because MLD in large part determines the nutrient supply to the upper waters, primary production in this region also exhibits strong seasonal variability. In summer, strong solar radiation stratifies upper ocean layers, resulting in a shallow MLD, nutrient limitation, and reduced primary production in most of the basin.

Off Hainan and along the coast southeastern Vietnam, however, the strong southwesterly winds generate upwelling during summer, thus enhancing primary production (Ning et al., 2004, Su and Pohlmann, 2009, Cai et al., 2015). In winter, in addition to the strengthened surface cooling, the monsoon enhanced wind mixing deepens the mixed layer, thus entraining nutrient-laden subsurface water to the surface (Zhang et al., 2019). This results in the enhanced surface water photosynthesis and higher concentrations of Chl-a (Tseng et al., 2005). Therefore, new primary production (NPP) in the SCS basin is typically uppermost in winter, when the greatest amount of nitrate is injected into the euphotic zone (Chen, 2005).

Intraseasonal events are also important. Cyclonic eddies (Tang et al., 1999, Wang et al., 2016, Zhang et al., 2019), typhoons (Liu, H. et al., 2013, Liu, K.K. et al., 2013) and internal waves (Pan et al., 2012) may all serve to upwell nutrient-rich deep water to the upper ocean, thereby stimulating the phytoplankton growth and increasing the primary production. Elevated primary production has also been observed in the northern SCS during aerosol (e.g., dust) deposition events (Wong et al., 2002, Zhang et al., 2019). These events directly load bio-nitrogen (N) from external sources into the SCS (Krishnamurthy et al., 2007, Kim et al., 2014, Zhang et al., 2019) while also supplying iron (Fe) to stimulate nitrogen fixation (Wong et al., 2002, Wong et al., 2007). The extent to which these processes and their seasonal, intraseasonal, and interannual variability impact the planktonic foraminifera shell fluxes and geochemistry is yet not fully understood.