Warm, not super-hot, temperatures in the early Eocene subtropics

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Abstract The early Eocene (ca. 55–48 Ma) encompasses one of the warmest intervals of the past 65 my and is characterized by an unusually low equator-to-pole thermal gradient. Recent proxy studies suggest temperatures well in excess of 30 C even at
  GEOLOGY, August 2011 771 ABSTRACTThe early Eocene (ca. 55–48 Ma) encompasses one of the warm-est intervals of the past 65 m.y. and is characterized by an unusu-ally low equator-to-pole thermal gradient. Recent proxy studies sug-gest temperatures well in excess of 30 °C even at high latitudes, but conflicting interpretations derived from different types of data leave considerable uncertainty about actual early Eocene temperatures. A robust comparison among new paleotemperature proxies may pro-vide insight into possible biases in their temperature estimates, and additional detail on the spatial distribution of temperatures will further resolve the early Eocene meridional temperature gradient. We use a suite of paleotemperature proxies based on the chemistry of bivalve shell carbonate and associated sedimentary organic mat-ter from the United States Gulf Coastal Plain to constrain climate at a subtropical site during this key interval of Earth history. Oxygen isotope and clumped isotope analyses of shell carbonate and two tet-raether lipid analyses of sedimentary organic carbon all yield tem-peratures of ~27 °C. High-resolution, intraannual oxygen isotope data reveal a consistent, large range of seasonal variation, but clumped isotope data suggest that seasonality is due primarily to precipitation, not to temperature. These paleotemperature estimates are 2–3 °C warmer than the northern Gulf of Mexico today, and generally con-sistent with early Eocene temperature estimates from other low and mid-latitude locations, but are significantly cooler than contempora-neous estimates from high southern latitudes.INTRODUCTION Paleotemperature estimates from a variety of geochemical and pale-ontological proxies indicate that the early Eocene encompasses the warm-est climatic conditions of the Cenozoic (the past 65 m.y.). Intervals of extreme warmth are of great interest, as they provide analogs for a future greenhouse world. Recent estimates suggest temperatures approaching or in excess of 30 °C not only in low latitudes (Pearson et al., 2007), but also middle (Zachos et al., 2006) and high latitudes (Bijl et al., 2009; Creech et al., 2010; Hollis et al., 2009), implying very low meridional gradients. Concerns over potential biases related to preservation of carbonate micro-fossils (Pearson et al., 2001; Schrag, 1999) and temperature significance of the newer organic proxies (e.g., see discussion in Liu et al., 2009), how-ever, leave some uncertainty about the actual magnitude and distribution of paleotemperatures in the early Eocene (Huber, 2008). Expanding the spatial coverage of paleotemperature estimates and corroborating inferred values with multiple proxies will enable an assessment of potential biases in any one proxy, refine interpretations of mean annual temperature (MAT) at the surface, and ultimately allow for a better understanding of processes responsible for maintaining Eocene warmth and equability.We use multiple geochemical proxies applied to the shells of bivalve mollusks and their associated sediment to assess paleotemperatures on a subtropical continental margin. Samples come from shallow-marine sedi-ments of the early Eocene upper Hatchetigbee Member of the Hatchetig-bee Formation (ca. 54–52 Ma; see the GSA Data Repository 1  for stron-tium isotope age constraints) in the Gulf Coastal Plain of the southeastern United States (for details, see the Data Repository), at a paleolatitude of ~30°N (Müller et al., 2011). Sediments are unconsolidated and fos-sils are in excellent condition, retaining their srcinal aragonite (see the Data Repository). Earlier oxygen isotope work revealed unexpectedly low seasonal δ 18 O minima within individual shells, suggesting summer tem-peratures of ~37 °C and leading the authors to propose a scenario of sea-sonal freshwater mixing to locally decrease water δ 18 O values and bias calculated temperatures (Ivany et al., 2004). These depleted δ 18 O values, however, could also be consistent with a mean tropical sea-surface tempera-ture of ~31 °C proposed by Pearson et al. (2007) for that same time interval. We combine stable oxygen isotope, clumped isotopes, and strontium iso-tope analyses of shell carbonate with tetraether lipid proxies (TEX 86 , BIT [branched and isoprenoid tetraether], MBT/CBT [methylation of branched tetraethers/cyclization of branched tetraethers]) from organic matter in sedi-ment enclosed by articulated shells to deconvolve the competing effects of temperature and salinity on the δ 18 O record and produce reliable estimates of winter, summer, and MATs during this interval of exceptional warmth. METHODS The shells of two individuals of the bivalve Venericardia hatcheplata  from each of seven stratigraphic horizons within the Hatchetigbee Forma-tion were cut along the maximum growth axis from umbo to ventral mar-gin, and polished. Because these bivalves grow more or less continuously throughout the year, high-resolution sequential microsampling of accre-tionary shell carbonate can recover seasonal variation (Ivany et al., 2004). Multiple, sequential, growth-band–parallel samples were milled from each couplet of light and dark growth bands in the inner shell layer using a Merchantek (New Wave) MicroMill. Within each bivalve, at least three full years of growth were sampled wherever possible. Aragonite powders were analyzed for stable carbon and oxygen isotope values at the Univer-sity of Michigan’s Stable Isotope Laboratory using MAT 251 and MAT 253 mass spectrometers. Resulting oxygen isotope values were converted to paleotemperatures using the corrected biogenic aragonite temperature equation of Grossman and Ku (1986; Kobashi and Grossman, 2003) and assuming seawater δ 18 O is the latitude-corrected ice-free marine value ( − 0.36‰ at 30°N; Zachos et al., 1994).Aliquots from light and dark growth band increments of four veneri-cards, as well as two bulk-shell samples that ranged across multiple years of growth, were analyzed for clumped isotopes ( ∆ 47 ). Carbonate samples were measured for ∆ 47  values using methodology described elsewhere (Affek and Eiler, 2006; Ghosh et al., 2006; Huntington et al., 2009). Tem-perature estimates were calculated from ∆ 47  values using the calibration of Ghosh et al. (2006).Sediments enclosed within six pairs of articulated valves of Veneri-cardia , two from each of three horizons, were analyzed for tetraether lipid distributions. TEX 86  values were calculated following Schouten et al. (2002), and converted to sea-surface temperatures (SSTs) using the non-linear calibration of Liu et al. (2009). The BIT index, a measure for terres-trial organic matter input to sediments, was calculated following Hopmans et al. (2004). Terrestrial mean annual temperature (MAT) estimates were calculated from MBT and CBT values following Weijers et al. (2007).   Geology , August 2011; v. 39; no. 8; p. 771–774; doi:10.1130/G32054.1; 3 figures; Data Repository item 2011229.© 2011 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.*Current address: Department of Geoscience, University of Wisconsin−Madison, Madison, Wisconsin 53706, USA; E-mail: keatingbiton@wisc.edu. 1 GSA Data Repository item 2011229, background information, methods, Tables DR1−DR4 (data tables), and Figures DR1−DR3, is available online at www.geosociety.org/pubs/ft2011.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. Warm, not super-hot, temperatures in the early Eocene subtropics Caitlin R. Keating-Bitonti 1 *, Linda C. Ivany 1 , Hagit P. Affek 2 , Peter Douglas 2 , and Scott D. Samson 1 1 Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA 2 Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520-8109, USA  772 GEOLOGY, August 2011 RESULTS δ 18 O values vary approximately sinusoidally between − 5.7‰ and − 1.4‰ along all 14 shell transects, reflecting seasonal cycles (Fig. 1). Dark (translucent) bands correspond to more negative δ 18 O values, sug-gesting, in the absence of other evidence, formation during warm seasons. Assuming normal marine salinity, calculated winter minimum tempera-tures are consistent within and among shells of all horizons, with an over-all mean of 26.6 ± 1.1 ° C (±1 standard deviation) (Fig. 2; Table DR1 in the Data Repository). Inferred summer maximum temperatures, however, are very warm and significantly more variable, ranging between 31 and 43 ° C (Fig. 2; Ivany et al., 2004).Clumped isotopes analyses suggest a MAT of 26.5 ± 1.0 °C (±1 standard error, n = 10) (Fig. 2; Table DR2). Mean summer and winter samples produce paleotemperatures of 25.5 ± 2.1 °C and 27.1 ± 1.7 °C, respectively (see the Data Repository). Paired clumped isotope analyses of light and dark growth bands within shells reveal summer and winter tem-peratures that are within error (based on the analytical error for individual samples; Fig. 1; Table DR2). The variability among shells is larger (22.4 ± 1.9 °C to 31.1 ± 0.7 °C) and likely reflects a combination of analytical uncertainty and real temperature variability on time scales longer than the lifetime of an individual bivalve. These results argue that negative δ 18 O values from shell carbonate do not reflect seasonally warm temperatures, but rather a substantial seasonal contribution of 18 O-depleted freshwater. Seasonal clumped isotope temperatures likely reflect integrated seasonal means, as the relatively large sample size required does not allow for the capture of seasonal temperature extremes. The δ 18 O values of the samples used for clumped isotopes analysis illustrate that, while micromilled car-bonate from light and dark growth bands generally differ by ~2.5‰, cor-responding samples measured for clumped isotopes differ by 1.5‰–2‰.Temperatures derived from tetraether lipid analyses are within error or slightly higher than those from clumped isotopes. TEX 86  indicates tem-peratures between 27 and 29 °C (Fig. 2; see the Data Repository). BIT values are between 0.3 and 0.4 (see the Data Repository), indicating a coastal setting with significant terrestrial organic matter input (Hopmans et al., 2004), consistent with the inference of seasonal precipitation and runoff drawn from seasonally depleted shell isotope values and corrobo-rated by the abundance of terrestrial vascular plant organic debris in the sediments. These relatively high BIT values indicate a possible source of uncertainty in TEX 86  derived temperatures because isoprenoid tetraether lipids derived from soils can influence TEX 86  values in settings with large terrestrial organic matter inputs (Weijers et al., 2006). Nevertheless, the TEX 86  derived temperatures basically agree with those from isotopic analy-ses of shell carbonates, and since the latter are not affected by organic inputs, the agreement suggests that TEX 86  reliably reflects the temperature of coastal waters in this setting. The MBT/CBT paleotemperature proxy, however, specifically targets terrestrially derived organic matter, and sug-gests soil paleotemperatures between 25 and 28 °C (Fig. 2; see the Data Repository). Although a large uncertainty exists concerning the source regions for soil-derived branched tetraether lipids in marine sediments, consistency in inferred temperatures suggests that nearby continental MATs were similar to coastal sea-surface temperatures. DISCUSSION Good agreement among paleotemperatures derived from winter δ 18 O, clumped isotopes, TEX 86 , and MBT/CBT indicates that the MAT on the U.S. Gulf Coast during the early Eocene was ~27 °C. Latest Paleocene TEX 86 derived temperatures from a core in nearby southeastern Missis-sippi (van Roij, 2009) are in line with those presented here. Winter and summer temperatures are statistically indistinguishable using clumped isotopes, implying that the range of seasonal temperature variation is sig-nificantly lower than suggested by δ 18 O data. The very negative “sum-mer” δ 18 O values instead indicate that the primary manifestation of the seasonal cycle was related to precipitation, making it the wet season. Sea-sonally resolved δ 18 O values from more typical marine facies elsewhere in the upper Hatchetigbee Member and underlying Bashi Marl Member of the Hatchetigbee Formation yield low seasonal ranges and similar “win-ter”/dry season values, reinforcing this interpretation (Ivany et al., 2003; -6-5-4-3-2-1-6-5-4-3-2-1-6-5-4-3-2-1-6-5-4-3-2-1-6-5-4-3-2-1-6-5-4-3-2-1-6-5-4-3-2-1-6-5-4-3-2-1-6-5-4-3-2-1-6-5-4-3-2-1-6-5-4-3-2-1-6-5-4-3-2-1-6-5-4-3-2-1-6-5-4-3-2-1 1a/1c 1b2a* 2b3a 3b4a 4b5a 5b6a7a6b7b 000000000000003. 654321 43214321 54321 54321 654321 6 7 854321 Distance (mm) Distance (mm)          δ    1   8    O   (   V   P   D   B ,   ‰   )          δ    1   8    O   (   V   P   D   B ,   ‰   )          δ    1   8    O   (   V   P   D   B ,   ‰   )          δ    1   8    O   (   V   P   D   B ,   ‰   )          δ    1   8    O   (   V   P   D   B ,   ‰   )          δ    1   8    O   (   V   P   D   B ,   ‰   )          δ    1   8    O   (   V   P   D   B ,   ‰   )   T  e  m  p  e  r  a   t  u  r  e   (   °   C   )   T  e  m  p  e  r  a   t  u  r  e   (   °   C   )   T  e  m  p  e  r  a   t  u  r  e   (   °   C   )   T  e  m  p  e  r  a   t  u  r  e   (   °   C   )   T  e  m  p  e  r  a   t  u  r  e   (   °   C   )   T  e  m  p  e  r  a   t  u  r  e   (   °   C   )   T  e  m  p  e  r  a   t  u  r  e   (   °   C   ) 45403530254540353025454035302545403530254540353025454035302520454035302545403530254540353025454035302545403530254540353025454035302545403530250 Figure 1. Variation in 18 O of Venericardia   carbonate sequentially sampled in direction of shell growth. Shells are arranged in strati-graphic order. Inferred temperatures are reported assuming con-stant 18 O W  value of –0.36‰. Shaded gray bars indicate dark (trans-lucent) growth bands. Horizontal solid, dashed, and dotted lines denote mean clumped isotope-derived “winter,” “summer,” and bulk temperatures, respectively. Analytical error for clumped isotope data is ~±2 °C. Plot 1a/1c represents 18 O C  values of venericard 1a and clumped isotope values of venericard 1c. Note different scale of y axis for plot 2a*. (See the Data Repository [see footnote 1] for all clumped isotope data and details.) VPDB—Vienna Peedee belemnite.  GEOLOGY, August 2011 773 Kobashi et al., 2001). A MAT of ~27 °C, minimal temperature seasonality, and significant seasonal precipitation is in good accord with paleobotani-cal data supporting tropical to paratropical conditions in the early Eocene Gulf Coast (Fricke and Wing, 2004; Harrington, 2003; Wolfe, 1978; Wolfe and Dilcher, 2000).The multiple paleotemperature proxies used in this study enable a comparison among them. This is the first time that the clumped iso-topes technique has been tested on ancient or modern biogenic carbonates where other independent proxies have been simultaneously applied. That clumped isotope temperatures generally agree with conventional oxygen isotope–derived temperatures determined a priori with an independent estimate of seawater composition, as well as with two organic geochemi-cal proxies, demonstrates conclusively that the technique can be effec-tive on carbonates as old as 50 m.y., despite concerns about the effects of diagenesis (Came et al., 2007; Dennis and Schrag, 2010). The Gulf Coast, however, is perhaps a best-case scenario, with sediments that have never been deeply buried or tectonically deformed. The narrower range of temperatures derived from the organic molecular proxies is likely due to attenuation of short-term variability by time averaging associated with sedimentation. The accretionary, in situ growth of shell carbonate instead records intraannual and interannual variation in conditions.Paleotemperature estimates derived from TEX 86  are often somewhat warmer than corresponding δ 18 O or Mg/Ca-derived temperatures, raising concerns about temperature calibration, especially in high-latitude settings during greenhouse intervals (Huber, 2008). Several calibrations have been developed for deriving SST from TEX 86  (Kim et al., 2010, 2008; Liu et al., 2009; Schouten et al., 2002) and it is not yet clear which is the most rel-evant relationship. The processes controlling the temperature dependence of archaeal lipid distributions are poorly understood, thereby preventing a process-based choice between calibrations. Early linear calibrations are likely problematic, as reflected by their inability to fit values at high-SST locations (Kim et al., 2010; Liu et al., 2009; Schouten et al., 2003), because physiological responses to temperature change are typically non-linear. We therefore prefer the nonlinear, reciprocal relationship reported by Liu et al. (2009) that provides the simplest relationship covering the entire temperature range of modern core top data. This calibration results in lower temperatures than those derived through the logarithmic calibra-tion of Kim et al. (2010) (Fig. 3). Our TEX 86 derived temperatures using the Liu et al. (2009) calibration are consistent with those from clumped isotopes, lending support for that calibration (Fig. 3, inset). Although the MBT/CBT index proxy for continental temperatures is new and requires further testing and calibration (Weijers et al., 2006), its agreement with the SST proxies in this case provides support for its application in low-lying coastal sediments rich in terrestrial soil organic matter.A mean early Eocene paleotemperature of ~27 °C on the U.S. Gulf Coast is only 2–3 °C warmer than modern sea-surface MAT in the north-ern Gulf of Mexico (Levitus and Boyer, 1994) and is consistent with non–Paleocene-Eocene Thermal Maximum temperatures observed at a mid-latitude site to the north (New Jersey; Sluijs et al., 2007; Zachos et al., 2006). These temperatures are, however, as much as 4 °C cooler than recently reported early Eocene values at both lower and higher latitudes (Fig. 3). Tropical temperatures of ~31 °C (Tanzania; Pearson et al., 2007) are not necessarily in conflict with temperatures of 27 °C in the subtrop-ics, but temperatures warmer than this at higher latitudes (New Zealand and the Tasman Rise; Bijl et al., 2009; Creech et al., 2010; Hollis et al., 2009) are more difficult to reconcile without considering potential unrec-ognized biases among proxies. Multiproxy temperature estimates from the U.S. Gulf Coast suggest warm, but not super-hot, paleotemperatures with low seasonal variation in the U.S. subtropics, provide further evidence for a reduced meridional temperature gradient during the early Eocene, and highlight the anomalous warmth implied by temperature reconstructions from the southwest Pacific Ocean. ACKNOWLEDGMENTS We thank Bruce Wilkinson, Andrew Haveles, Mark Pagani, Jocelyn Sessa, and Ethan Grossman for discussion, and Emily Feinberg, Bryan Sell, Aaron Satkoski, Tathagata Dasgupta, and Gerry Olack for assistance in the lab. Several anonymous reviewers provided feedback that significantly improved the manuscript. Nan Arens examined organic matter in Hatchetigbee Bluff sediments. Lora Wingate (University of Michigan Stable Isotope Laboratory) ran stable isotope analyses. Work was sup-ported by grants from Sigma Xi, the Paleontological Society, and the Department of Earth Sciences at Syracuse University to Keating-Bitonti, and by grants from the American Chemical Society’s Petroleum Research Fund and the National Science Foundation to both Ivany and Affek. Affek and Douglas thank the Earth System Center for Stable Isotope Studies of the Yale Institute for Biospheric Studies. 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