Elsevier

Geochimica et Cosmochimica Acta

Volume 170, 1 December 2015, Pages 69-93
Geochimica et Cosmochimica Acta

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

https://doi.org/10.1016/j.gca.2015.08.003Get rights and content

Abstract

Within the Gulf of Mexico (GOM), the Bryant Canyon is exceptional in that rapid sedimentation sustained by high rates of terrigenous sediment delivery and surface marine productivity make this basin an excellent recorder of paleoenvironmental and paleoclimatic conditions. We present a new 21-kyr record of sea surface temperature (SST) and local salinity changes from the NW GOM (core JPC-26) approximated from combined Globigerinoides ruber morphotype-specific δ18O and Mg/Ca, as well as Ba/Ca, which reflect the subtropical GOM hydrographic dynamics and their relationship to both Mississippi River discharges and climate evolution during the last deglaciation. Overall, the reconstructed SST and SSS-related patterns reveal notable variations in amplitude between the analyzed morphotypes, adding valuable insights to previously published G. ruber (w, mixed) GOM records. Especially during the deglaciation, Bryant Canyon meltwater flooding events (BCDFs) and associated sea surface freshening seem to be more pronounced than the SST reduction. Our Mg/Ca-derived SST records of both morphotypes (G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.)) show comparable general trends, but with important SST differences (ΔT). We interpret down-core ΔT as a record of changing upper water column hydrography, with particular influence from the deglacial meltwater (BCDFs) or Holocene (BCHFs) flooding events. During the warm intervals, the deeper, thicker and probably more seasonally persistent mixed layer led to more uniform thermal conditions for both G. ruber morphotypes and therefore to a minimum ΔT. On the contrary, during the cold and low salinity periods, the shallower mixed layer favored more habitat divergence, and caused a maximum thermal gradient in the well-stratified upper water column. Overall, this supports the notion that G. ruber s.s. is consistently calcifying in warmer waters than G. ruber s.l., either due to a shallower depth habitat or to a more summer-weighted seasonal distribution. Moreover, intra-specific paired δ13C and Ba/Ca differences show that G. ruber s.l. is more sensitive to river water influence than G. ruber s.s., due to its ability to change its depth habitat and therefore exploit optimal (temperature, salinity, productivity, stratification) conditions during the low-salinity events. Overall, our observations support the current practice of treating the two G. ruber morphotypes separately and further illustrate the necessity to map both their spatial and temporal distribution.

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

References (113)

  • J.E. Ferguson et al.

    Systematic change of foraminiferal Mg/Ca ratios across a strong salinity gradient

    Earth Planet. Sci. Lett.

    (2008)
  • P. Hamilton

    Deep currents in the Gulf of Mexico

    Prog. Oceanogr.

    (2009)
  • J.S. Hanor et al.

    Non-conservative behavior of barium during mixing of Mississippi River and Gulf of Mexico waters

    Earth Planet. Sci. Lett.

    (1977)
  • B. Hönisch et al.

    Planktic foraminifers as recorders of seawater Ba/Ca

    Mar. Micropaleontol.

    (2011)
  • B. Hönisch et al.

    The influence of salinity on Mg/Ca in planktic foraminifers – evidence from cultures, core-top sediments and complementary δ18O

    Geochim. Cosmochim. Acta

    (2013)
  • J.P. Kennett et al.

    Late Pleistocene paleoclimatology, foraminiferal biostratigraphy and tephrochronology, western Gulf of Mexico

    Quat. Res.

    (1972)
  • J.P. Kennett et al.

    The last deglaciation in Orca Basin, Gulf of Mexico: high-resolution planktonic foraminiferal changes

    Palaeogeogr. Palaeoclimatol. Palaeoecol.

    (1985)
  • B. Kisakürek et al.

    Controls on shell Mg/Ca and Sr/Ca in cultured planktonic foraminiferan, Globigerinoides ruber (white)

    Earth Planet. Sci. Lett.

    (2008)
  • A. Kuroyanagi et al.

    Vertical distribution of living planktonic foraminifera in the seas around Japan

    Mar. Micropaleontol.

    (2004)
  • A. Kuroyanagi et al.

    The occurrence of two genotypes of the planktonic foraminifer Globigerinoides ruber (white) and paleoenvironmental implications

    Mar. Micropaleontol.

    (2008)
  • D.W. Lea et al.

    Controls on magnesium and strontium uptake in planktonic foraminifera determined by live culturing

    Geochim. Cosmochim. Acta

    (1999)
  • A. Leventer et al.

    Dynamics of the Laurentide ice sheet during the last deglaciation: evidence from the Gulf of Mexico

    Earth Planet. Sci. Lett.

    (1982)
  • H.-L. Lin et al.

    Seasonal variations of modern planktonic foraminifera in the South China Sea

    Deep-Sea Res. II

    (2007)
  • H.L. Lin et al.

    Seasonal variation of planktonic foraminiferal isotopic composition from sediment traps in the South China Sea

    Mar. Micropaleontol.

    (2004)
  • L. Löwemark et al.

    A test of different factors influencing the isotopic signal of planktonic foraminifera in surface sediments from the northern South China Sea

    Mar. Micropaleontol.

    (2005)
  • T.A. Mashiotta et al.

    Glacial interglacial changes in Subantarctic sea surface temperature and d18O-water using foraminiferal Mg

    Earth Planet. Sci. Lett.

    (1999)
  • J.C. Montero-Serrano et al.

    Sedimentary evidence of deglacial megafloods in the northern Gulf of Mexico (Pigmy Basin)

    Quat. Sci. Rev.

    (2009)
  • J.C. Montero-Serrano et al.

    Changes in precipitation regimes over North America during the Holocene as recorded by mineralogy and geochemistry of Gulf of Mexico sediments

    Global Planet. Change

    (2010)
  • F.E. Müller-Karger et al.

    Pigment distribution in the Caribbean Sea: observations from space

    Prog. Oceanogr.

    (1989)
  • R. Muscheler et al.

    Geomagnetic field intensity during the last 60,000 years based on 10Be and 36Cl from the summit ice cores and 14C

    Quat. Sci. Rev.

    (2005)
  • L. Numberger et al.

    Habitats, abundance patterns and isotopic signals of morphotypes of the planktonic foraminifer Globigerinoides ruber (d’Orbigny) in the eastern Mediterranean Sea since the Marine Isotopic Stage 12

    Mar. Micropaleontol.

    (2009)
  • D. Nürnberg et al.

    Interacting loop current variability and Mississippi River discharge over the past 400 kyr

    Earth Planet. Sci. Lett.

    (2008)
  • J.N. Richey et al.

    Ecological controls on the shell geochemistry of pink and white Globigerinoides ruber in the northern Gulf of Mexico: implications for paleoceanographic reconstruction

    Mar. Micropaleontol.

    (2012)
  • B. Schmuker et al.

    Planktic foraminifers and hydrology of the eastern and northern Caribbean Sea

    Mar. Micropaleontol.

    (2002)
  • D.P. Schrag et al.

    The oxygen isotopic composition of seawater during the Last Glacial Maximum

    Quat. Sci. Rev.

    (2002)
  • T. Sionneau et al.

    Clay mineral distributions in and around Mississippi River watershed and Northern Gulf of Mexico: sources and transport patterns

    Quat. Sci. Rev.

    (2008)
  • T. Sionneau et al.

    Provenance of freshwater pulses in the Gulf of Mexico during the last deglaciation

    Quat. Res.

    (2010)
  • J.D. Stanford et al.

    Sea-level probability for the last deglaciation: a statistical analysis of far-field records

    Global Planet. Change

    (2011)
  • S. Steinke et al.

    Proxy dependence of the temporal pattern of deglacial warming in the tropical South China Sea: toward resolving seasonality

    Quat. Sci. Rev.

    (2008)
  • P. Aharon

    Meltwater flooding events in the Gulf of Mexico revisited: implications for rapid climate changes during the last deglaciation

    Paleoceanography

    (2003)
  • P. Anand et al.

    Variability of Mg/Ca and Sr/Ca between and within the planktonic foraminifers Globigerina bulloides and Globorotalia truncatulinoides

    Geochem. Geophys. Geosyst.

    (2005)
  • P. Anand et al.

    Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series

    Paleoceanography

    (2003)
  • J.I. Antonov et al.
  • S. Barker et al.

    A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry

    Geochem. Geophys. Geosyst.

    (2003)
  • A.W.H. et al.

    Distribution and ecology of living planktonic foraminifera in surface waters of the Atlantic and Indian Oceans

  • B.E. Bemis et al.

    Reevaluation of the oxygen isotopic composition of planktonic foraminifera: experimental results and revised paleotemperature equations

    Paleoceanography

    (1998)
  • K. Bice et al.

    Application of secondary ion mass spectrometry to the determination of Mg/Ca in rare, delicate, or altered planktonic foraminifera: examples from the Holocene, Paleogene, and Cretaceous

    Geochem. Geophys. Geosystem.

    (2005)
  • G. Bond et al.

    A pervasive millennial scale cycle in North Atlantic Holocene and glacial climates

    Science

    (1997)
  • A.H. Bouma et al.

    Initial Reports Deep Sea Drilling Project

    (1996)
  • Briggs J. C. and Ficke J. F. (1978) Quality of rivers of the United States, 1975 water year. U.S. Geol. Surv. Open File...
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