The Consequences for Strontium

PROCEEDINGS OF THE SECOND JOINT SYMPOSIUM ON THE NATURAL HISTORY AND GEOLOGY OF THE BAHAMAS Edited by Tina M. Niemi and Kathleen Sullivan Sealey ORGANIZER: Troy A. Dexter Executive Director Gerace Research Centre University of The Bahamas San Salvador, The Bahamas 2020 iii Copyright 2020, Gerace Research Centre All rights reserved. No part of this work may be reproduced or transmitted in any form by any means, electronic or mechanical, including photocopying, recording, or any data storage or retrieval system without the express written permission of the Gerace Research Centre. ISBN: 978-0-935909-67-8 iv The 2nd Joint Symposium on the Natural History and Geology of The Bahamas SPELEOTHEM DEPOSITION IN EOGENETIC CARBONATES: THE CONSEQUENCES FOR STRONTIUM Nicole Ridlen1, John E. Mylroie1, Jason Polk2, and Joan R. Mylroie1 1Department of Geosciences, Mississippi State University, Mississippi State, MS 39762 2Department of Geography and Geology, Western Kentucky University, Bowling Green, KY 42101 ABSTRACT this rock contained 0.25 ppm Sr. The “Middle Ter- race,” younger mid-Pleistocene surface rock, is Eogenetic carbonate rocks develop from al- also calcitic, less than 5% aragonite, with 1.43 ppm lochems precipitated primarily as aragonite (e.g. Sr; two cave speleothems had Sr values of 0.30 and green algae, mollusks, corals) and high-Mg calcite. 0.31 ppm. The “Lower Terrace” surface rock, late (e.g. echinoderms, red algae). Relative to calcite, Pleistocene in age, was 40% aragonite and 60% strontium (Sr) preferentially enters the orthorhom- calcite, with a Sr value of 6.89 ppm; two cave spe- bic aragonite crystal lattice, giving eogenetic car- leothems had Sr values of 8.68 and 8.28 ppm. Uti- bonate rocks a higher Sr background than older tel- lizing caves from a single location removes any cli- eogenetic rocks, in which the aragonite has in- matic differences in calcite inversion and speleo- verted to calcite. Vadose speleothems (stalag- them deposition, leaving rock age as the major fac- mites) forming in caves in eogenetic carbonate tor in the Sr variation in these speleothems. In The rock should show a high Sr content. The Sr levels Bahamas, collection was done along a climatic gra- in the speleothems should decrease with rock age dient of decreasing rainfall, NW to SE, but the in- as the aragonite inverts and the Sr is lost. Climate ability to accurately determine host rock age made can also affect aragonite inversion, with wetter cli- the results here equivocal. mates leading to more rapid inversion. Strontium levels in speleothems have been used as a paleoclimate indicator, but the influence of eogenetic Sr INTRODUCTION has been under appreciated. This issue has become more important as oceanic island caves and spele- The presence of strontium (Sr) in carbonate othems hosted in eogenetic carbonates have re- rocks has long been documented (Kulp et al., 1962) cently been utilized as mid-ocean paleoclimate in- and has been recognized as a contributor to the dicators. trace element content of speleothems used for Curacao, in the southern Caribbean, con- paleoclimate reconstruction (e.g. Goede et al., tains a series of tectonically uplifted reef terraces 1998). Strontium in speleothems can originate of eogenetic carbonate rock. These carbonate from multiple sources although for the majority of rocks contain flank margin caves in which speleo- current research, the Sr is assumed to be infiltrated them growth is active. Surface samples and spele- from a surface source (e.g. Roberts et al., 1998; othems from each terrace show a progressive loss Finch et al., 2003; White, 2004; van Beynen et al., of Sr in each higher, and therefore older, terrace. 2008; Sinclair et al., 2012). In younger, eogenetic The “Higher Terrace,” older mid-Pleistocene age carbonates, the mineralogy is aragonite and the surface rock, is calcitic, less than 5% aragonite, aragonite crystal structure accommodates Sr in re- with 0.44 ppm Sr; one speleothem from a cave in placement of Ca (White, 2004; Hill and Forti, 119 The 2nd Joint Symposium on the Natural History and Geology of The Bahamas 1997). Over time, aragonitic carbonate rocks will ate units in the study area of Curaҫao in the Neth- invert to the more stable polymorph mineral of cal- erlands Antilles (Figure 1A and 1B), are often ter- cite (Hill and Forti, 1997). This inversion releases raced; where the oldest unit is at higher elevations Sr to the vadose zone, as the Sr cannot be accom- and the youngest at lower elevations (Schellmann modated in the calcite crystal lattice of the host et al., 2004). These units are composed of coral reef rock. carbonates that were originally composed of arag- The carbonate host rocks that are mineralogi- onite (Schellmann et al., 2004). The inversion of cally aragonite will naturally have a greater Sr con- aragonite to calcite is not only time dependent, but tent than those rocks that are calcite. The older eo- is also dependent on the flux of meteoric water genetic rock units of the Caribbean typically have through the limestone and thus a climatic signature. a higher calcite to aragonite ratio than those that are Abaco Island in The Bahamas is the wettest island younger (Gaffey et al., 1995). It can then be sur- in the Bahamian archipelago (Figure 1C); Long Is- mised that younger carbonates in the Caribbean land and San Salvador, The Bahamas are two of the should have a greater amount of Sr contribution drier islands in the archipelago, having negative from the host rock into those speleothems than spe- water budgets (Sealey, 1990). However, the islands leothems that form in older carbonates that are in- are assumed to have carbonates of the same ap- verted to calcite. While Sr does not fit easily into proximate age, so any difference in aragonite in- the calcite crystal lattice, it can be incorporated into version degree should be a climatic signal as op- the growth bands on the growing stalagmite to posed to an age signal as for the island of Curaҫao. leave a record of the dripwater geochemistry. This interpretation also assumes past climates are Uplifted island carbonates, such as the carbon- similar to those seen today. Figure 1: A. Caribbean and North Atlantic locations of the study sites. B. Curaçao is the middle of the three ABC Islands. C. Abaco, San Salvador, and Long Island trend across a climatic gradient (modified from Whitaker and Smart, 1997). 120 The 2nd Joint Symposium on the Natural History and Geology of The Bahamas Speleothem research has become an integral available, so time constraints remain unknown. Fu- part of paleoclimate reconstruction because cave ture work plans to complete a more robust data set deposits preserve the conditions that were present and provide better interpretations using additional at the time of speleothem deposition. As cited ear- trace elements and age constraints. lier, most speleothem research seeks to interpret the local Earth surface conditions at the time of deposition, such as temperature, rainfall, vegeta- METHODS tion. However, the state of the host rock itself can also be preserved in the speleothem record. The research presented here sought to answer Fieldwork on the island of Curaҫao took three questions: 1) does the Sr content of Caribbean place in December, 2012, while the fieldwork on speleothems have a direct relationship with the age the islands of Abaco, Long Island, and San Salva- of the host rock at the time of current speleothem dor took place in June, 2013. Prior fieldwork on precipitation? 2) do speleothems from older host Curaҫao conducted by Kambesis et al. (2015) and rock contain less Sr than speleothems from Sumrall et al. (2016) provided information on younger host rock in the same climatic setting? caves in these field areas that allowed for planning and, 3) will speleothems record the change in Sr of sample collection. Due to the delicate nature of concentration of eogenetic carbonates as a faster collecting and exporting cave samples, permission depletion in climates of higher precipitation as op- for six stalagmites (two each from three caves in posed to drier climates? each of the terrace levels) was obtained from the Traditional work on speleothems has been con- Carmabi Foundation for Curaҫao which is equiva- centrated on those located in much older geologic lent to their national park service. In addition, per- settings where the limestones are all calcite and Sr mission for collecting multiple stalagmites from is more likely to be an infiltrate from an external caves on Abaco Island, San Salvador Island, and source. The work on speleothems in younger, Sr- Long Island was obtained from the Bahamas Envi- enriched host rocks will need to consider the pos- ronment, Science and Technology (BEST) Com- sibly significant amount of host-rock sourced Sr mission. within the speleothem record. This could affect the The Curaҫao caves identified for collection interpretations of paleoclimatic data from the use were Lardem Cave in the Upper Pleistocene (MIS of speleothems in young carbonates. By determin- 5e) lower terrace; Raton Cave in the middle terrace ing the correlation of host-rock age with strontium of mid-Pleistocene age; and Hato Cave within the concentration in the speleothem in contrast with older and uppermost high terrace, also of mid- the variation of strontium in differing climatic set- Pleistocene age. These caves are all flank margin tings, a calibration could be developed to better in- caves that developed in the freshwater lens (Myl- terpret earlier paleoclimatic studies in young car- roie and Mylroie, 2007; Kambesis et al., 2015). bonates. For each terrace, the time window of speleogenesis Recently, speleothem research has been shift- was restricted to when the terrace was elevated and ing to oceanic islands to obtain speleothem climate exposed to allow meteoric catchment to create a records in mid-ocean areas not available on conti- freshwater lens, but before the terrace was further nents (e.g.

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