Biotic and geochemical (δ18O, δ13C, Mg/Ca, Ba/Ca) responses of Globigerinoides ruber morphotypes to upper water column variations during the last deglaciation, Gulf of Mexico
Introduction
The tropical Western Hemisphere Warm Pool (WHWP), which encompasses the Caribbean Sea and the Gulf of Mexico (GOM), is an important heat and moisture source for climate in the North Atlantic region (Wang and Enfield, 2001), and therefore acts as a key regulator of the subpolar North Atlantic oceanography and climate in NW Europe. The GOM is a key area for global thermohaline circulation (THC), global heat exchange and the global climate system (Broecker, 1991), because its hydrographic characteristics (high sea surface temperature (SST) and salinity (SSS) values) partly control Gulf Stream physical properties. Thermal variability determines the amount of water vapor exported and the intensity of storm tracks in the region (Oglesby et al., 1989), and S changes of the surface currents entering via the Yucatan Channel (Weldeab et al., 2006) into the GOM also have a direct impact on its hydrography.
However, during the last deglaciation, GOM hydrography was further episodically impacted by freshwater outputs from the Mississippi River resulting from intermittent melting of the Laurentide Ice Sheet (LIS) (Flower et al., 2004, Aharon, 2006). The direct effect of these freshwater flooding events on the GOM paleoceanography has long attracted the attention of researchers, and therefore has been studied intensely in cores from different sub-basins of the GOM (e.g. Orca Basin, Pigmy Basin, DeSoto Canyon, Bryant Canyon – Aharon, 2003, Aharon, 2006, Flower et al., 2004, LoDico et al., 2006, Tripsanas et al., 2007, Tripsanas et al., 2013, Nürnberg et al., 2008, Montero-Serrano et al., 2009, Montero-Serrano et al., 2010, Montero-Serrano et al., 2011, Sionneau et al., 2010, Williams et al., 2010) through foraminiferal isotope and trace element analyses. However, knowledge on the different sensitivities of planktonic foraminifera to S changes associated with these meltwater pulses is limited. A recent multi-species study of Bahr et al. (2013) in core tops from the southeastern Caribbean Sea suggests that Globigerinoides ruber is the best-suited species for reconstructing modern SSS changes. Nevertheless, although they documented intra-specific divergences in G. ruber s.l. and G. ruber (p) Ba/Ca and δ18O signatures, they did not conclude that either morphotype was especially geochemically sensitive in recording Orinoco River discharges. Moreover, several studies have shown that the two G. ruber morphotypes (G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.) are genetically distinct (Kuroyanagi et al., 2008), and they have different depth preferences (Kuroyanagi and Kawahata, 2004, Lin and Hsieh, 2007) reflected in their distinctive Mg/Ca (Steinke et al., 2005) and isotopic (Wang, 2000, Lin et al., 2004, Kawahata, 2005, Löwemark et al., 2005) signatures. However, a very recent work performed on sediment trap, core-top and late Holocene down-core material from the northern GOM (Thirumalai et al., 2014) seems not to confirm that morphotype variability has a significant effect on G. ruber (w) geochemistry. These conflicting results, despite the different sample numbers and size fractions analyzed, emphasize the need for further comparison and precise quantification of the potential bias on each geochemical proxy.
With this motive, we present a continuous LGM-Holocene morphotype-specific micropaleontological and geochemical study, by analyzing G. ruber s.s. and G. ruber s.l., following the definition of Wang (2000). The main purpose of this study is to test whether geochemical analyses effectively reflect the ecologic variability observed for the 2 G. ruber morphotypes, and further constrain the application of the different morphotypes for reconstructing past GOM hydrography. We also performed statistical analysis to quantify the degree of the potential bias that may occur in each geochemical proxy. Scanning Electron Microscope (SEM) analyses were also performed to better constrain the diagenetic alteration and/or dissolution of G. ruber shells that could potentially affect Mg/Ca and Ba/Ca proxies. Our paleoceanographic study focuses on morphotype-specific Mg/Ca-SST and the stable oxygen isotope composition of seawater (δ18OSW, approximating SSS) reconstructions over the last termination (T1), based on core JPC-26 from the Bryant Canyon in the northwestern GOM. Because riverine water has a much higher concentration of dissolved barium (Ba2+) (Weldeab et al., 2007), we also measured Ba/Ca ratios in both G. ruber morphotypes, to test the ability to reflect the Mississippi River inputs. The changes in water column hydrography in the GOM across the last deglaciation and through the Holocene provide multiple scenarios of freshwater incursions (meltwater deglacial discharge and Holocene runoff due to enhanced precipitation), water column stratification, decreased salinity, and enhanced primary productivity across which to examine the records of the two morphotypes. Overall, in this study we build upon previous studies by presenting the differential response degree between the G. ruber morphotypes on these sea surface properties. Issues associated with depth habitat and ecological or seasonal preferences are further discussed, as these are fundamental to the reliable interpretation of the δ18O and Mg/Ca signals. Finally, although at lower resolution, these paleoceanographic and paleoclimatic data are further compared with previously published regional (GOM δ18OIVF-SW and Mg/Ca-SSTs) climate records and local productivity patterns (Globigerina bulloides % abundance) during T1 to understand processes involved in driving the hydrography.
Section snippets
Oceanographic setting and site location
The GOM is a semi-enclosed basin at the northwestern edge of the tropical Atlantic Ocean (Fig. 1), with hydrographic properties that result mainly from the complex interactions between the inflow of warm tropical waters originating from the Caribbean Sea through the Loop Current (LC), the evaporation/precipitation (E/P) budget, and North American rivers freshwater supply. The LC is the most prominent surface circulation feature in the GOM, due to the formation of large anticyclonic eddies (
Core selection and lithology
Core JPC-26 was selected on the basis of (1) absence of discernible erosional unconformities, as judged by high resolution seismics, (2) uniform lithology of foraminifera-rich hemipelagic muds, and (3) presence of biostratigraphic markers for the Y1/Y2, Y/Z and Z1/Z2 chronozones during the early deglaciation, the Pleistocene/Holocene and Mid-Late Holocene transitions, respectively (Kennett et al., 1985, Flower and Kennett, 1990), indicating that the core covers the complete deglaciation
Planktonic faunal pattern
Planktonic foraminifera are abundant and well preserved in all the samples from JPC-26. The qualitative analysis of the planktonic foraminifera allow the identification of 22 species lumped into 15 groups: Globigerinoides ruber (w), Globigerinoides ruber (p), Globigerinoides sacculifer group, Globigerinoides conglobatus, Globigerinoides tenellus, Globoturborotalita rubescens, Globigerina bulloides, Globogerinella aequilateralis group, Globorotalia scitula, Globorotalia menardii, Globorotalia
Statistical constraints
All proxies analyzed here (Mg/Ca, δ18O, δ13C, Ba/Ca) display statistically significant differences between G. ruber morphotypes during T1, with the most pronounced of these in the stable isotope proxies (mean Δδ18Os.s.-s.l = 0.59 ± 0.08‰ and Δδ13Cs.s.-s.l = 0.61 ± 0.06‰ compared to the relevant values of 0.27 ± 0.22‰ (δ18O)/-0.25 ± 0.19‰ (δ13C) of Steinke et al. (2005) and 0.21 ± 0.21‰ (δ18O)/−0.28 ± 0.29‰ (δ13C) of Wang (2000) from the South China Sea). However, these offsets are in contrast to the recent
Conclusions
In this study, we present a downcore comparison of δ13C, δ18O, Mg/Ca, and Ba/Ca records for two morphotypes of the planktonic foraminifer G. ruber (s.s. and s.l.) in order to discretely test existing, conflicting results on these morphotypes, and furthermore to gain insight into their use as paleoceanographic and paleoclimatic recorders in the GOM. Statistical and regression analyses performed on paired G. ruber morphotype-specific δ18O, δ13C, Mg/Ca and Ba/Ca from core JPC-26 revealed
Acknowledgments
The authors are grateful to Laura Rodríguez Sanz and Ignacio Villarroya for kind assistance during the trace element analyses. Special thanks are due to Dr. Aristomenis Karageorgis for providing the study material. Professor Dr. Matthew Schmidt is warmly thanked for his constructive suggestions that helped to improve the manuscript, and Lecturer Dr. Margarita Dimiza and Dr. Elizabeth Stathopoulou are thanked for assistance and technical support during SEM analyses. Constructive comments by
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