Planktic foraminifera-bound organic nitrogen isotopic composition in contemporary water column and sediment trap
Introduction
Planktic foraminifera are important recorders of surface ocean processes. In multi-chambered planktic foraminifera, chambers are formed by calcite precipitation on both sides of a chamber-shaped organic sheet (OS)—organic material secreted by living foraminifera during ontogenesis (Hemleben et al., 1977). Once incorporated, the organic matrix, i.e., OS, is protected from degradation during sinking and burial (King and Hare, 1972). Recently, a new proxy, which relies on measuring the nitrogen isotopic composition of organic nitrogen bound to the OS trapped within planktonic foraminifera tests (or shells), i.e., foraminifera-bound δ15N (FB-δ15N), has been developed to reconstruct past changes in surface ocean N2 fixation in tropical regions (Ren et al., 2009, Ren et al., 2012a, Ren et al., 2017, Straub et al., 2013, Wang et al., 2018) and surface ocean nitrate consumption in high-latitude regions (Martínez-García et al., 2014; Ren et al., 2015). To date, investigations on FB-δ15N in living foraminifera have been reported only in two studies (Ren et al., 2012b; Smart et al., 2018). Using net-tow samples, Smart et al. (2018) showed that there is no systematic offset between FB-δ15N and tissue δ15N in living foraminifera from the Sargasso Sea, supporting the hypothesis that FB-δ15N reflects δ15N changes in the upper ocean nitrogen inventory. In addition, FB-δ15N values of shallower-dwelling species were reported to be lighter than those of deeper-dwelling species, which indicates an increase in δ15N of their diets with depth (Ren et al., 2012b; Smith et al., 2018). Based on a comparison of surface sediment FB-δ15N with nitrate δ15N of subsurface water, FB-δ15N of euphotic-dwelling species, e.g., Globigerinoides sacculifer (Brady 1877), Globigerinoides ruber (d’Orbigny 1839), Orbulina universa (d’Orbigny 1839), were reported to record shallow thermocline nitrate δ15N in tropical/subtropical oligotrophic regions (Ren et al., 2009, Ren et al., 2012a). However, ambiguities still exist regarding the interpretation of FB-δ15N paleo-records. For example, seasonal variations of planktic FB-δ15N reflect changes in upward nitrate supply and euphotic-zone ammonium recycling (Ren et al., 2012b; Smart et al., 2018), complicating the interpretation of euphotic FB-δ15N in tracing N2 fixation. In addition, increasing FB-δ15N was observed from the net-tow to the sediment trap to the surface sediment in the Sargasso Sea (Smart et al., 2018), suggesting possible alteration of the ontogenic FB-δ15N signal during sinking and burial.
During the final stage of their lifecycle, most planktic foraminifera (e.g., G. sacculifer and O. universa) experience gametogenesis, a process during which some species migrate to a deeper (or shallower) location in the water column to form gametogenic calcite and release their gametes for reproduction (Caron et al., 1990, Erez et al., 1991, Schiebel and Hemleben, 2005). Gametogenic calcification leads to the enrichment of δ18O and Mg in G. sacculifer tests (Spero and Lea, 1993, Nürnberg et al., 1996, Anand et al., 2003) and the depletion of Mg in Globorotalia truncatulinoides tests relative to the pre-gametogenic tests (Eggins et al., 2003). Recently, Smith et al. (2018) speculated that gametogenic calcification does not require chamber-shaped OS as a template, suggesting that gametogenesis may not affect ontogenic FB-δ15N; however, this speculation has not yet been validated. G. sacculifer is one of the most abundant species in tropical and subtropical open oceans (Schmuker and Schiebel, 2002, Lin and Hsieh, 2007) and its carbonate test is widely used as an archive for paleoclimate and paleoceanography research. During gametogenesis, an elongated sac-like final chamber is added to the outside of the G. sacculifer ontogenic calcite test. Two morphotypes, one with a sac-like final chamber (gametogenic carbonate test) and one without a sac-like final chamber (pre-gametogenic carbonate test, also named as T. sacculifer), can thus be identified for G. sacculifer tests collected from both the water column and sea floor sediments. This provides an opportunity to directly compare FB-δ15N values in the tests of G. sacculifer with-sac and G. sacculifer without-sac and to validate the aforementioned presumption by Smith et al. (2018).
In this study, planktic foraminifera were collected by plankton tows in the South China Sea (SCS) and the Bay of Bengal. Planktic foraminifera G. sacculifer tests with-sac and without-sac were separated prior to the FB-δ15N analysis. The FB-δ15N analysis was also carried out for planktic foraminifera species, including G. sacculifer with-sac, Globigerinella siphonifera (d’Orbigny 1839, previously named as Globigerinella aequilateralis), and O. universa, collected from a sediment trap deployed in the northern SCS. This study aims to verify whether gametogenic calcifying encapsulates organic N with a δ15N that is different from the ontogenic FB-δ15N signal and to better understand the planktic FB-δ15N signals among different species in tropical oceans.
Section snippets
Study area
The SCS is a semi-closed marginal sea that lies in the western tropical-to-subtropical Pacific Ocean, exchanging water with the western Pacific through the Luzon Strait (Gan et al., 2016, Li et al., 2017). In general, the SCS has a shallower thermocline and higher nitrate concentrations than the western Pacific at the same water depth (Yang et al., 2017). In addition, the northern SCS nitrate δ15N profiles below the shallow thermocline are spatially uniform (Ren et al., 2017, Yang et al., 2017
Sample information
Living foraminifera were collected by deploying a 0.44 m2, 100 µm mesh plankton net over the upper 60–200 m of the water column in the SCS and the Bay of Bengal from R/V Shiyan 3 and R/V Shiyan 1, respectively (see details in Table 1). The sediment trap, named 09Trap F7, was deployed at 1460 m water depth in the northern SCS Xisha Trough (17°25.909′ N, 110°53.160′ E, Fig. 1) from June 16, 2009 to July 12, 2010, with a collecting area of 0.5 m2. Plankton tow and sediment trap samples were
Results
Availability of foraminifera tests prevented the duplicate analyses except for G. sacculifer (with-sac) from the plankton tow site 07E409, G. sacculifer (with-sac) and G. siphonifera from the sediment trap 09Trap F7. In Figs. 2 and 3, error bars indicate the standard deviation (1 sigma) of duplicate analyses.
FB-δ15N of G. sacculifer from plankton tows
Planktic foraminifera G. sacculifer is a symbiont-bearing species living in the upper euphotic zone (Anderson and Bé, 1976, Jørgensen et al., 1985). The ontogenic calcification depth or the averaged dwelling depth of G. sacculifer is shallower than 50 m in the modern northern SCS (Lin et al., 2011). Hence, the G. sacculifer carbonate test carries mixed-layer environmental signals in the SCS (Lin et al., 2011, Ren et al., 2012a, Ren et al., 2017). During the early stage of gametogenesis, a
Conclusions
Planktic foraminifera tests from plankton tows in the SCS and the Bay of Bengal and from sediment traps in the northern SCS were analyzed for FB-δ15N. Main conclusions are as follows:
- (1)
G. sacculifer tests with-sac and G. sacculifer without-sac collected from plankton tows exhibit similar FB-δ15N values, suggesting that the thermocline δ15N signal is not incorporated into the carbonate tests during gametogenic calcification. Thus, G. sacculifer tests with-sac and without-sac can be binned together
Acknowledgements
We thank Dr. Stephan Steinke for his help with foraminifera test picking in College of Ocean and Earth Science (Xiamen University) and Dr. Xianhui Wan for GC-IRMS technical assistance. We also thank Dr. Willie Soon for English language editing. This manuscript was greatly improved by comments from four anonymous reviewers and Editor Roger Francois. This is MEL (State Key Laboratory of Marine Environmental Science) publication 2018314. This work was supported by the National Natural Science
References (43)
Gametogenic calcification in a spinose planktonic foraminifer, Globigerinoides sacculifer (Brady)
Mar. Micropaleontol.
(1980)- et al.
Experimental determination of the ontogenetic stable isotope variability in two morphotypes of Globigerinella siphonifera (d'Orbigny)
Mar. Micropaleontol.
(1998) - et al.
Effects of gametogenesis on test structure and dissolution of some spinose planktonic foraminifera and implications for test preservation
Mar. Micropaleontol.
(1990) - et al.
Nitrogen isotope fractionation by oceanic zooplankton
Deep Sea Res. Part A
(1989) - et al.
Mg/Ca variation in planktonic foraminifera tests: implications for reconstructing palaeo-seawater temperature and habitat migration
Earth Planet. Sci. Lett.
(2003) - et al.
Circulation and oxygenation of the glacial South China Sea
J. Asian Earth Sci.
(2017) - et al.
Seasonal variations of modern planktonic foraminifera in the South China Sea
Deep Sea Res. Part II
(2007) - et al.
Stable isotopes in modern planktonic foraminifera: sediment trap and plankton tow results from the South China Sea
Mar. Micropaleontol.
(2011) The seasonal succession of modern planktonic foraminifera: sediment traps observations from southwest Taiwan waters
Cont. Shelf Res.
(2014)- et al.
Monsoon-forced chlorophyll distribution and primary production in the South China Sea: observations and a numerical study
Deep Sea Res. Part I
(2002)