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Boron Proxies In Paleoceanography And Paleoclimatology
Bärbel Hönisch, Stephen M. Eggins, Laura L. Haynes, Katherine A. Allen, Katherine D. Holland, Katja Lorbacher
This edition first published 2019
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Library of Congress Cataloging‐in‐Publication Data
Names: Hönisch, Bärbel, 1974– author. | Eggins, Stephen Malcolm, author. | Haynes, Laura Louise, 1991– author. | Allen, Katherine Ann, author. | Holland, Katherine Davina, author. | Lorbacher, Katja, author.
Title: Boron proxies in paleoceanography and paleoclimatology / Bärbel Hönisch, Stephen Malcolm Eggins, Laura Louise Haynes, Katherine Ann Allen, Katherine Davina Holland, Katja Lorbacher.
Description: Hoboken, NJ : John Wiley & Sons, 2018. | Series: New analytical methods in earth and environmental science series | Includes bibliographical references and index. |
Identifiers: LCCN 2018032341 (print) | LCCN 2018057134 (ebook) | ISBN 9781119010647 (Adobe PDF) | ISBN 9781119010623 (ePub) | ISBN 9781119010630 (hardcover)
Subjects: LCSH: Paleoceanography. | Seawater–Carbon dioxide content. | Paleoclimatology. | Boron–Isotopes.
Classification: LCC QE39.5.P25 (ebook) | LCC QE39.5.P25 H66 2018 (print) | DDC 551.46–dc23
LC record available at https://lccn.loc.gov/2018032341
Cover Design: Wiley
Cover Image: Courtesy of Bärbel Hönisch
Atmospheric carbon dioxide levels are rising at a pace that may be unprecedented in Earth history, and it is unclear how much this will warm our planet and whether marine life can adapt to acidifying oceans. To understand where our climate and oceans are headed, we seek information from Earth history, for instance through the geochemical signals stored in the fossil remains of marine organisms. The boron isotope proxy for past seawater pH was first introduced two decades ago, but its application has only started to gain momentum over the past decade, when the biological and inorganic constraints on boron incorporation into marine carbonates became better understood, studies confirmed the potential for reconstructing atmospheric pCO2 beyond ice cores, and new analytical techniques were developed. The related B/Ca proxy is based on the same principles as the boron isotope proxy, but B/Ca was traditionally considered a temperature proxy in corals, and its potential for reconstructing pH had not been explored until about a decade ago. Several complications have been encountered over the years, and selecting the best samples for answering a specific question, sample preparation and analysis are complexities that have restricted analyses to a handful of laboratories worldwide. Premature interpretation of unsuitable sample material has created confusion about whether the proxies are reliable, or which technique should be used. Therefore, as more scientists embark on characterizing past ocean acidity and atmospheric pCO2, it is important to provide a resource that helps to educate and train geoscientists in the opportunities and complications of this method. We hope that this book will provide a useful guideline for the interested researcher.
We would like to thank the many people that helped us write this book – students, friends, and colleagues who provided data, discussed aspects of boron and carbonate chemistry with us, taught us how to read the chemical parlance of the past century, or how to implement up and coming methods of this century, who simply shared their enthusiasm, and encouragement for boron and the product in hand, and provided comments on earlier drafts. Too many to list, but we are deeply indebted to Michael Henehan and Claire Rollion‐Bard for reviewing this book and providing many valuable comments and suggestions. We do not agree on all aspects discussed herein, but we all concur that there are many opportunities to strengthen boron proxies even further, and that there are many avenues to reach this goal. In addition, we would like to specifically thank the following friends and colleagues for their support (in alphabetical order): Jelle Bijma, Oscar Branson, Aaron Celestian, Rob DeConto, Jesse Farmer, Mathis Hain, Gil Hanson, Gary and Sidney Hemming, Damien Lemarchand, Chiara Lepore, Tim Lowenstein, Alberto Malinverno, Gianluca Marino, Miguel Martínez‐Botí, Vasileios Mavromatis, Helen McGregor, Oded Nir, Mo Raymo, Andy Ridgwell, Dana Royer, Mats Rundgren, Abhijit Sanyal, Gavin Schmidt, Paolo Stocchi, Daniel Storbeck, Taro Takahashi, Joji Uchikawa, Avner Vengosh, Richard Zeebe. And finally, we are grateful to the research stations on Santa Catalina Island and One Tree Island, where this book took its first steps.
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Abstract
This chapter presents a brief introduction to marine carbonate chemistry systematics, including definitions of different pH scales. As a starting point, published estimates of Pleistocene and Cenozoic pCO2 reconstructions from boron isotopes and B/Ca ratios in planktic foraminifera are shown in the context of ice core records and reconstructions from terrestrial leaf stomata and marine alkenones. These published boron proxy records form the foundation for discussing boron proxy systematics and sensitivity studies presented in the following chapters.
Keywords: atmospheric pCO2; seawater carbonate chemistry; seawater pH; pH scales
It has been known since the early studies of Arrhenius (1896) that anthropogenic emissions of carbon dioxide from fossil fuel burning and land use changes will warm our planet, but direct evidence for increasing atmospheric pCO2 levels emerged only in 1958, when Charles Keeling started continuous measurements at the Mauna Loa Observatory on Hawaii and initially observed an average annual value of 315 parts per million (ppm) (Keeling et al. 1976). These atmospheric pCO2 levels varied seasonally, steadily increased year upon year and were finally put into perspective when Raynaud and Barnola (1985) presented the first pCO2 measurements from Antarctic ice cores, which revealed pre‐anthropogenic background levels as low as 260 ppmv (parts per million by volume). Subsequent studies expanded the ice core records to 800 000 years ago and constrained the pre‐industrial range of atmospheric pCO2 to 172–300 ppmv, together with concomitant Antarctic temperature fluctuations of ~12 °C (Barnola et al. 1987; Jouzel et al. 1987; Lüthi et al. 2008; Petit et al. 1999; Siegenthaler et al. 2005). In 2014 atmospheric pCO2 hit 400 ppm for the first time (Dlugokencky and Tans 2017) and levels are projected to climb to 420–940 ppm by the end of this century, depending on future emissions (Figure 1.1).
While discussion of the consequences of rising atmospheric pCO2 initially concentrated on global warming, research over the past two decades has increasingly addressed the dissolution of CO2 in seawater and its consequences for marine life. Briefly, as CO2 dissolves in the ocean, it hydrates and reacts with water to form carbonic acid, which then dissociates into bicarbonate, carbonate, and hydrogen ions according to the following reactions:
The more CO2 dissolves, the more hydrogen ions are created but these ions do not immediately accumulate, as they are buffered by the carbonate ions already in solution:
However, a small fraction of the resulting bicarbonate ions will dissociate, ultimately increasing the hydrogen ion concentration and therefore the acidity of seawater (i.e. lowering pH):
A detailed description of marine carbonate chemistry systematics and calculations can be found in Zeebe and Wolf‐Gladrow (2001); here we will limit the discussion to a few basic details. The reactions between carbonate and hydrogen ions are governed by dissociation constants (K1 and K2), which depend on the thermodynamic seawater properties pressure (p), temperature (T) and salinity (S). The associated shift in carbonate ion speciation is shown in Figure 1.2, which displays the relative concentrations of [CO2], [HCO3−] and [CO32−] versus seawater‐pH at typical surface (T = 25 °C, S = 35, and p = 1 bar) and deep ocean conditions (T = 4 °C, S = 34.8, p = 401 bar). In contrast, the sum of all dissolved inorganic carbon (DIC) species and their alkalinity (i.e. the sum of their charges) are independent of T, S, and p when expressed in gravimetric units (i.e. μmol kg−1, as opposed to the volumetric μmol l−1). Because these six parameters are interrelated, the entire carbonate system can be determined if two of its components, in addition to temperature, salinity, and pressure, are known. Several programs facilitate computation of the carbonate system; see Further Reading for details.
One aspect that requires specific attention is the choice of pH scale. Four scales have been defined, the National Bureau of Standards (NBS), free hydrogen, seawater, and total scale; they differ in the chemical composition of their respective reference material and pH values determined for identical solutions differ by up to 0.15 units (Table 1.1). While this pH difference may appear small, it has significant consequences for carbon system calculations, as demonstrated in Table 1.1. For example, assuming the same T, S, p, pH, and DIC value to calculate pCO2, but with pH defined on different scales, calculated pCO2 differs by >150 μatm. Such large differences are inacceptable for carbon system determinations and must be avoided by all means. Fortuitously, pH scales are interrelated and values can be converted (see Zeebe and Wolf‐Gladrow 2001), but this is only possible if studies cite the pH scale used. Because the boron equilibrium constants are reported for the total scale (Dickson 1990; Millero 1995), this book will present all data on the total scale.
Table 1.1 Definitions of pH scales, differences in scale‐specific pH values in solutions of the same composition, and differences in pCO2 calculated from solutions of similar composition but assuming pH = 8.10 for all four pH‐scales.
Scale | Definition | pH value at TA = 2400 μmol kg−1, DIC = 2100 μmol kg−1, T = 25 °C, S = 35, p = 1 bar | pCO2 at pH = 8.10, DIC = 2100 μmol kg−1, T = 25 °C, S = 35, p = 1 bar |
NBS (μmol kg −1 H2O) | pHNBS = −log aH+ | 8.162 | 513 |
Free (μmol kg −1 SW) | pHF = −log [H+]F | 8.133 | 477 |
Total (μmol kg −1 SW) | pHT = −log ([H+]F+[HSO4−]) | 8.025 | 363 |
Seawater (μmol kg −1 SW) | pHSWS = −log ([H+]F+[HSO4−] + [HF]) | 8.016 | 354 |
Calculations performed using the CO2SYS program (version 2.1) by Pierrot et al. (2006) with K1 and K2 according to Lueker et al. (2000), KSO4 according to Dickson (1990) and total [B] after Lee et al. (2010).
Modern surface ocean pH is ~8.1 (total scale, TS), which is already ~0.1 pH units lower compared to the preindustrial, when atmospheric pCO2 was ~120 ppm lower compared to today (Figure 1.1). Surface ocean pH continues to drop by ~0.002 units annually (Takahashi et al. 2014) and anthropogenic CO2 slowly enters the intermediate and deep ocean via thermohaline circulation (Feely et al. 2004; Khatiwala et al. 2012; Sabine et al. 2004). Although the incremental accumulation of hydrogen ions resulting from dissolution of CO2 will not actually turn seawater acidic (i.e. pH will not drop below 7), the trend towards decreasing pH has been termed Ocean Acidification (Caldeira and Wickett 2003). Depending on the source and extent of future anthropogenic carbon emissions, surface seawater pH is projected to decrease by an additional 0.1–0.7 pH units by the year 2200 (Figure 1.1). Laboratory experiments with various marine organisms and observations of naturally acidified ecosystems have highlighted the vulnerability of marine life to ocean acidification, but also the diversity of the biotic response (for a review see Doney et al. 2009).
Despite a wealth of experimental and observational work, projections of future ecosystem changes in the warming and acidifying ocean suffer from limited diversity and typically short duration of laboratory experiments, a shortcoming that can be compensated by the study of the geological record (e.g. Hönisch et al. 2012). Similarly, improving estimates of future warming requires better estimates of climate sensitivity, and the geological record offers a multitude of opportunities to study the interplay of CO2 and temperature (Foster et al. 2017, PALEOSENS‐project‐members 2012). While polar ice provides the best archive for past CO2 concentrations, continuous ice core records are currently limited to the past 800 000 years (Lüthi et al. 2008). Horizontal drilling into Antarctic blue ice has recovered isolated sections ~1 million years old (Higgins et al. 2015) and ~2.7 million years old (Yan et al. 2017), but the prospect of a continuous vertical record may not exceed 1.5 million years (Fischer et al. 2013). The study of geological archives therefore requires the use of proxies, i.e. measurable stand‐ins for environmental parameters that can no longer be measured directly. CO2‐ proxies have been developed for the terrestrial and the marine realm, and include the stomata density of fossil leaves, the carbon isotopic composition (δ13C) of marine biomarkers, and the boron isotopic composition and B/Ca ratios recorded in foraminifer shells, among others (e.g. Beerling and Royer 2011; Foster et al. 2017). Figures 1.3 and 1.4 display a selection of reconstructions over the past 800 000 and 65 million years, respectively. The functioning of the systematics, advantages, and shortcomings of the proxies displayed in these figures have been reviewed in Royer et al. (2001a) and Allen and Hönisch (2012). Because this book focuses on boron proxies, we will only mention the systematics of other proxies briefly.
Of the proxies shown, only the stomata (breathing cells) of vascular land plants are directly related to atmospheric pCO2 – the stomatal index decreases as atmospheric pCO2 increases, such that water loss via evaporation can be minimized when CO2 is abundant (e.g. Royer et al. 2001b), but see also Franks et al. (2014) for additional environmental and stomatal anatomy controls on leaf gas exchange. Alkenone pCO2 estimates are based on the carbon isotope fractionation that occurs during photosynthesis performed by marine haptophytes, where δ13Calkenone is inversely related to aqueous [CO2], but also depends on algal growth rate (i.e. nutrient supply) and cell geometry (e.g. Henderiks and Pagani 2007). As such, alkenone reconstructions require a few auxiliary data, including estimates of δ13C of DIC, temperature, nutrients, and cell geometry (e.g. Zhang et al. 2013), all of which can be estimated from respective marine proxy records. Boron isotopes and B/Ca ratios in planktic foraminifer shells are not directly related to pCO2 but rather to seawater acidity, and thus require a second parameter of the carbonate system to estimate pCO2 via pH. The second parameter is often given by an assumption of total alkalinity, which changes little on Pleistocene time scales, but is more uncertain on multi‐million year time scales (Caves et al. 2016; Ridgwell 2005; Tyrrell and Zeebe 2004). Boron proxy‐to‐pCO2 translations also require estimates of temperature and salinity, in addition to knowledge of the boron isotopic composition (δ11Bsw), boron and calcium concentrations of seawater. The details of these parameters and translations will be explained later in this book, for now it suffices to say that pCO2 reconstructions from proxies are more complicated than the extraction of actual CO2 from air trapped in polar ice. However, despite the complexity of the respective translation process, validation of proxy estimates relative to ice core pCO2 (Figure 1.3) shows convincing results. Going further back in time, pCO2 estimates from different proxies show relatively consistent values until ~40 Ma, but diverge greatly during the early Eocene and Paleocene, with δ11B estimates showing the highest pCO2 values (Figure 1.4).
While these proxy estimates have greatly enhanced our understanding of Earth's climate system, the uncertainties associated with all of these pCO2 estimates preclude accurate estimates of climate sensitivity (PALEOSENS‐project‐members 2012). Improvements have been made over the past few years but are still needed for all proxies. In particular, some of the boron proxy records shown in Figures 1.3 and 1.4 are no longer considered scientifically sound, and we will discuss individual boron proxy records in detail. However, this book will not only focus on atmospheric pCO2. Estimates of seawater pH in coral reefs and carbon storage in the deep ocean are all aspects that contribute to our understanding of the marine carbon system, climate, and ecosystem dynamics. These properties can be reconstructed with boron proxy estimates in marine carbonates as different as shallow and deep‐water coral skeletons, planktic, and benthic foraminifer shells, brachiopod shells, and inorganic precipitates. In addition to pCO2 estimates beyond ice cores, boron proxies thereby provide a plethora of opportunities to decipher the causes of past carbon cycle variations and their effect on marine ecosystems.
We acknowledge the World Climate Research Programme's Working Group on Coupled Modeling, which is responsible for CMIP, and we thank the climate modeling groups (listed at http://cmip‐pcmdi.llnl.gov/cmip5/docs/CMIP5_modeling_groups.pdf) for producing and making available their model output. For CMIP, the U.S. Department of Energy's Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals.
CO2 in seawater: Equilibrium, Kinetics, Isotopes, by Zeebe and Wolf‐Gladrow, Elsevier Oceanography Series, Volume 65, 360 pp., eBook ISBN: 9780080529226, 2001 – the resource for all questions on ocean carbonate chemistry
Ocean carbonate chemistry calculation programs can be found at: