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Nova Hedwigia, Beiheft 148, p. 101–112 C Stuttgart, February 2019 Potential use of chrysophyte cyst morphometrics as a tool for reconstructing ancient lake environments Peter A. Siver Department of Botany, Connecticut College, New London, CT 06320, USA; [email protected] With 8 fi gures Abstract: Chrysophycean algae produce a siliceous resting stage, the cyst, within the confi nes of the cell as a response to environmental or population stimuli. Modern cysts range in diameter from approximately 3 to 35 μm, the external morphology is species-specifi c, and the diff erent morphotypes can serve as valuable bioindicators in paleolimnological investigations. In this study, massive numbers of fossil cysts were obtained from an extensive core representing an Arctic maar lake in northern Canada that existed during the globally-warm middle Eocene. The exquisite preservation of microfossils makes the locality an especially valuable site for understanding impacts of warming on Arctic water bodies. Since the vast majority of fossil cysts in the core could not be linked to modern analogs, cyst morphology could not be easily used to infer past conditions. Instead, a method was devised to extract the cysts from the core and then measure morphometric characters on hundreds of specimens using a FlowCam. The goal was to characterize the size distributions of cysts in each stratum, and use the information to aid in reconstructing lake history. Over 25,000 cysts ranging in diameter from 3 to 32 μm were measured from 33 strata, classifi ed into frequency distributions, and used to trace changes over the history of the ancient lake. The cyst size categories were found to be highly correlated with other biological proxies, and relationships between mean cyst diameter and overall cyst diversity were established. Findings indicate that cyst size classes are potentially valuable proxies for reconstructing historical conditions of aquatic habitats. Key words: cysts, Chrysophyceae, Eocene, FlowCam, Synurophyceae. Introduction The Chrysophyceae and Synurophyceae, commonly referred to as golden-brown algae, are diverse, cosmopolitan, and ecologically signifi cant groups of heterokont organisms that are especially important in freshwater ecosystems (Andersen, 2004; Kristiansen, 2005; Adl et al., 2012; Siver, 2015a). Species are mostly microscopic and may be planktonic or attached, autotrophic or heterotrophic, naked or with a cell covering, motile or non-motile, and the group embraces a wide diversity of vegetative forms (Kristiansen, 2005; Nicholls & Wujek, 2015; Siver, 2015a). The Synurophyceae include golden-brown organisms characterized by distinctive siliceous scales thatuncorrected_proof produce a highly organized covering around the cell (Siver, 2015a). Even though the Chrysophyceae and Synurophyceae are closely related, the precise phylogenetic relationship between the two groups of organisms requires further investigation (Andersen et al., 1999; Škaloud et al., 2013; Siver et al., 2015). Some molecular phylogenies conclude that the Synurophyceae forms a distinct class separate from the Chrysophyceae (Andersen, 2007; Yang et al., 2012), yet other works suggest the synurophytes are a monophyletic clade nested within the Chrysophyceae (del Campo & Massana, 2011; Škaloud et al., 2013). The Chrysophyceae (Nicholls & Wujek, 2015) and Synurophyceae (Siver, 2015a) are dis- tri buted worldwide from the tropics to high latitudes with highest diversities often associated with habitats that are slightly acidic, poorly buff ered, dilute, humic-stained and with low to moderate nutrient levels. Despite this generalization, both groups of organisms exist in virtually any water body and numerous species have optima at diff erent positions along environmental gradients making them excellent bioindicators (Siver & Smol, 1993; Stevenson & Smol, 2015). © 2019 J. Cramer in der Gebr. Borntraeger Verlagsbuchhandlung, Stuttgart, Germany www.borntraeger-cramer.de DOI 10.1127/nova-suppl/2019/115 1438-9134/2019/115 $ 2.75 eschweizerbart_xxx 102 Peter A. Siver As a result, these organisms have been eff ectively used to reconstruct acidity (e.g., Cumming et al., 1992; 1994; Facher & Schmidt, 1996; Siver et al., 1999), dissolved salts (Siver, 1993), nutrients (Siver & Marsicano, 1996; Kamenik et al., 2001), eff ects of deforestation (Lott et al., 1994), temperature (Siver & Hamer, 1992), and climate change (Kamenik & Schmidt, 2005; Arseneau et al., 2016). All golden-brown algae form cysts, hollow siliceous structures produced in response to environmental (Cronberg, 1986; Sandgren, 1981) or population (Sandgren, 1988) cues. Cysts range in diameter from ca. 3-35 μm, and vary immensely in surface design from smooth to highly ornamented (Duff et al., 1995; Pla, 2001; Wilkinson et al., 2001). Since each living cell can form a cyst, a trace of its existence can be deposited in the sediment record. Cysts are the most common type of microfossil representing chrysophytes and synurophytes in the geologic record, for which the oldest specimens are Late Triassic (ca. 230 million years before present; Ma) in age (Zhang et al., 2016). Hundreds of cysts have been described from an extensive diversity of modern aquatic environments (e.g., Duff et al., 1995; Pla, 2001), and cyst morphotypes have been used to successfully reconstruct conditions over the last several hundred years (Kamenik & Schmidt, 2005). A stunning array of exquisitely preserved cysts has been documented from a middle Eocene freshwater lake situated near the Arctic Circle in northern Canada known as the Giraff e Pipe locality (Siver & Wolfe, 2005; 2009). The idea has been put forward to use the numerous cysts to help reconstruct past conditions of the Giraff e lake since this water body existed under warm greenhouse conditions during the middle Eocene (Zachos et al., 2008). However, assigning specifi c conditions to these ancient fossil cysts has proven problematic because many fossil morphotypes lack modern analogs. The purpose of this study is to explore the possibility of using cyst diameter, instead of morphology, as a proxy of past environmental conditions for the Giraff e Pipe lake. To achieve this goal, three steps needed to be carried out. First, a technique was devised to extract cysts from the Giraff e mudstones. Second, a method to measure morphometric data on hundreds to thousands of cysts was perfected using a FlowCam. Third, changes in cyst size classes were traced over the sediments representing the ancient water body and correlated with the remains of other groups of organisms to further understand the utility of this metric in reconstructing lake history. Study site The Giraff e locality (65º N, 110º W) is a kimberlite diatreme crater that formed following a phreatomagmatic volcanic eruption (Heaman et al., 2004), subsequently infi lled with a sequence of Eocene lacustrine and then paludal sediments, and later became entombed with Neogene glacial deposits. The kimberlite pipe intruded through the Slave Province in the Northwest Territories of Canada approximately 48 Ma based on Rb-Sr model ages from kimberlitic phlogopite (Creaser et al., 2004; Siver & Wolfe, 2009). A 165-meter long drilled core was obtained from the crater by BHP Billitonuncorrected_proof Inc. during diamond exploration and later archived at the Canadian Geological Survey, Calgary, Canada. The core contains 113 m of stratifi ed organic sediment of middle Eocene age, including 68 m of lacustrine mudstone overlain with 45 m of terrestrial sediments. The lacustrine phase includes two air-fall tephra beds near the transition from aquatic to terrestrial remains (Siver & Wolfe, 2009). The Rb-Sr model age estimate is corroborated by the presence of pollen throughout the lacustrine and paludal sections that are diagnostic of the middle Eocene, including Platycarya swasticoides and Pistillipollenites mcgregorii (Rouse, 1977; Hamblin et al., 2003). Diameter-corrected and isothermal-plateau fi ssion tracking age estimates of the tephra beds center on 38 Ma (Doria et al., 2011), constraining the lacustrine sequence to late middle Eocene. The protracted regional tectonic and thermal stability of the Slave Craton, into which the Giraff e Pipe was intruded, has resulted in near-zero diagenetic alteration of the fossil content, as testifi ed by exquisite preservation, even at the sub-cellular level (Wolfe et al., 2006). eschweizerbart_xxx Reconstructing lake environments using cyst size 103 Methods Chips of mudstone (0.1–0.5 g) from the Giraff e core were oxidized using 30% H2O2 under low heat for one to four hours, rinsed four times with distilled water, and the resulting slurries stored in glass vials. For most samples this oxidation procedure resulted in separation of cysts from the mudstone matrix as observed with light microscopy. Sonication for 1 min could further separate the microfossils, but in most cases this was not needed. Other more resistant chips were subjected to a strong acid treatment involving a mixture of potassium dichromate and sulfuric acid after initial oxidation with H2O2. Signifi cant numbers of cysts were fully extracted from all strata examined in this study. Aliquots of each oxidized sample were air dried onto pieces of heavy duty aluminum foil. The aluminum foil samples were trimmed, attached to aluminum stubs with Apiezon® wax, coated with a mixture of gold and palladium for 2 min with a Polaron Model E sputter coater, and examined with a Leo 982 fi eld emission scanning electron microscope (FE-SEM) or an FEI Nova FE-SEM. Observations of the cysts