Reconstruction of Limnology and Microbialite Formation Conditions from Carbonate Clumped Isotope Thermometry V

Geobiology (2015), 13, 53–67 DOI: 10.1111/gbi.12121

Reconstruction of limnology and microbialite formation conditions from carbonate clumped isotope thermometry V. A. PETRYSHYN,1 D. LIM,2 B. L. LAVAL,3 A. BRADY,4 G. SLATER4 AND A. K. TRIPATI1,5 1Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA, USA 2Bay Area Environmental Research Institute, NASA-Ames Research Center, Moffett Field, CA, USA 3Department of Civil Engineering, University of British Colombia, Vancouver, BC, Canada 4School of Geography and Earth Sciences, McMaster University, Hamilton, ON, Canada 5Department of Atmospheric and Oceanic Sciences, Institute for the Environment and Sustainability, Institute of Geophysics and Planetary Physics, Los Angeles, CA, USA

ABSTRACT

Quantitative tools for deciphering the environment of microbialite formation are relatively limited. For example, the oxygen isotope carbonate-water geothermometer requires assumptions about the isotopic composition of the water of formation. We explored the utility of using ‘clumped’ isotope thermometry as a tool to study the temperatures of microbialite formation. We studied microbialites recovered from water depths of 10–55 m in Pavilion Lake, and 10–25 m in Kelly Lake, spanning the thermocline in both lakes. We determined the temperature of carbonate growth and the 18O/16O ratio of the waters that microbialites grew in. Results were then compared to current limnological data from the lakes to reconstruct the history of microbialite formation. Modern microbialites collected at shallow depths (11.7 m) in both lakes yield clumped isotope-based temperatures of formation that are within error of summer water temperatures, suggesting that clumped isotope analyses may be used to reconstruct past climates and to probe the environments in which microbialites formed. The deepest microbialites (21.7–55 m) were recovered from below the present-day thermoclines in both lakes and yield radioisotope ages indicating they primarily formed earlier in the Holocene. During this time, pollen data and our reconstructed water 18O/16O ratios indicate a period of aridity, with lower lake levels. At present, there is a close association between both photosynthetic and heterotrophic communities, and carbonate precipitation/microbialite formation, with biosignatures of photosynthetic inﬂuences on carbonate detected in microbialites from the photic zone and above the thermocline (i.e., depths of generally <20 m). Given the deeper microbialites are receiving <1% of photosynthetically active radiation (PAR), it is likely these microbialites primarily formed when lower lake levels resulted in microbialites being located higher in the photic zone, in warm surface waters.

Received 18 June 2014; accepted 17 September 2014

Corresponding authors: A. K. Tripati and V. A. Petryshyn. Tel.: +310 206 3531; fax: +310 825 2779; e-mails: [email protected] and [email protected]

called into question (e.g., Grotzinger & Rothman, 1996; INTRODUCTION Grotzinger & Knoll, 1999; Batchelor et al., 2000; Dupraz Understanding the environments in which stromatolites et al., 2006; McLoughlin et al., 2008; Petryshyn et al., and microbialites form has been a long-standing goal in 2012). Given this debate, developing new tools that can geobiology. Stromatolites are presumed to constitute some be used to study the conditions under which microbialite of the oldest evidence for life on Earth (and thus a target growth has occurred is of importance. for astrobiological study; e.g., Hofmann et al., 1999; A new and potentially powerful tool for the study of mi- Allwood et al., 2006) and probably are the ‘celebrity’ of crobialites is the carbonate clumped isotope thermometer. the microbialite world. However, the origins of stromato- This proxy is based on measuring the abundance of 13 18 16 lites are hotly debated, and their biogenicity is frequently C O OinCO2 produced by the dissolution of carbon-

© 2014 John Wiley & Sons Ltd 53 54 V. A. PETRYSHYN et al ate minerals in phosphoric acid, which is proportional to The majority of the microbialites from these lakes have a 13 18 16 2- the amount of C O O2 in the mineral (Ghosh et al., unique dendritic microstructure and are considered ana- 2006, 2007; Eiler, 2007; Tripati et al., 2010). The most logues for Early Cambrian reefs (Laval et al., 2000). The 12 16 16 common isotopic species of CO2 is C O O, with only microbialites in these lakes occur at a variety of depths, a small fraction of CO2 comprised of the heavier isotopo- from shallow/above the thermocline to below the thermo- logues. The most abundant of the minor isotopologues, by cline. Although the shallower microbialites are currently far, is the 13C18O16O species (Eiler, 2007). When a car- growing, there is some debate over the extent to which bonate mineral grows at thermodynamic equilibrium, the deepest microbialites (ca. 50 m) are actively accreting, there is an inverse correlation between the proportion of as radiometric ages for such samples from Pavilion Lake 13C-18O bonds and the temperature of precipitation; yield values of >6000 years before present (Table 1) (Brady thus, high temperatures of formation yield fewer et al., 2009). The microbialites from these two lakes pres- 13C-18O bonds than lower temperatures. Using this ent an ideal test case for the use of this new geochemical proxy, it is possible to estimate temperature independent proxy in the reconstruction of water temperatures, plane- of fluid composition and to routinely discern the temper- tary hydrology, geobiology, and paleoclimate. ature of carbonate precipitation to within 1–2°C in surfi- cial environments (Huntington et al., 2009; Eagle et al., PAVILION LAKE 2010, 2011, 2013a,b; Tripati et al., 2010, 2014; Thiagarajan et al., 2011). Pavilion Lake is located in the Marble Range, south-central Although this new and potentially powerful tool can be British Columbia, approximately 450 km northeast of Van- used in lacustrine settings to reconstruct terrestrial temper- couver (Fig. 1). It is an ultra-oligotrophic, dimictic freshwa- ature records and paleoenvironmental change, calibration ter lake with an area of 5.7 km 9 0.8 km, mean pH of 8.3, data are limited (Huntington et al., 2010; Csank et al., and a maximum depth of 60 m (Lim et al., 2009). The lake 2011; Hren & Sheldon, 2012). Temperature reconstruc- is located in a predominantly arid region; the average rainfall tions from lacustrine carbonates from sites with a strong in the area is 200 mm (Lim et al., 2009). seasonal cycle (i.e., at mid or high latitudes, or from high During summer, the lake thermally stratifies with a sur- elevation) rarely represent a time-averaged annual signal. face layer that is typically 19°C and a maximum thickness They typically reflect growth during a particular season that of 10 m (Fig. 2). Below the surface layer, lake temperature is likely summer or spring through autumn (Hren & decreases rapidly through a 2–3 m thick thermocline, Sheldon, 2012). below which temperature gradually decreases to 4°C at the Therefore, we applied the carbonate clumped isotope lake bottom, crossing 6°C at about 20 m depth. The d18O thermometer to freshwater microbialites of Pavilion Lake of waters in Pavilion Lake was measured from 2005–2008, and Kelly Lake in British Columbia, where vertical profiles with water samples collected from 7 to 24 m depth, across of temperature have been taken and water d18O has been the thermocline (Brady et al., 2010). Reported water d18O measured in samples taken from above and below the ther- values range from 10.7 to 11.4& (V-SMOW) mocline, and many dated microbialites have been collected. (Table 2).

Table 1 Summary of depth and apparent ages of Pavilion Lake microbialites (modiﬁed from Laval et al., 2000; and Brady et al., 2009). Samples in ﬁrst two columns are from Laval et al. (2000) which are U/Th series ages. Samples from Brady et al. (2009) are 14C age dates. Note that 14C ages may be affected by the incorporation of depleted carbon

Maximum age (years Current depth of microbialite (m) before present) Corrected age 14 C Age

20 1560 50 21.5 (inner microbialite) 2360 65 21.5 (outer microbialite) 1530 40 23 1710 50 26 1240 50 27-32 (base of microbialite) 12 300 1400 4430 1433 27-32 (base of microbialite) 3650 860 2240 1000 27-32 (center of microbialite) 8800 1165 4600 1175 27-32 (top of microbialite) 6100 1000 2570 1140 27-32 (top of microbialite) 8900 2100 3360 2100 29 1360 20 32 (center of microbialite) 6600 2700 2300 2000 33 4650 100 45 9550 30

Table 2 Hydrographic data for depths in Pavilion and Kelly Lakes where samples were taken. Oxygen isotope values for water are reported relative to V-SMOW standard and were measured over 3 year intervals and correspond to range observed, including samples taken across thermocline. Val- ues for Pavilion Lake are from Brady et al., 2010. Water temperatures are present-day summer values

Water Sample ID Depth (m) T(°C) Water d18O(&)

Pavilion Lake

RT 99 TP 062207 PL 35 FT 11.7 19 10.7 to 11.4 TP 180711-0:65 PL 65 ft 21.7 6 10.7 to 11.4 TP1707 11-81APL85 85 ft 28.3 4 10.7 to 11.4 2000806-28D-PV-DM-55m 55 4 10.7 to 11.4

o Kelly Lake 57 N KC 220711-35D 35 FEET 11.7 17 14.8 to 16.2 o 54 N KL 2007 11-65B 68 feet 22.7 4 14.8 to 16.2 51o N o 48 N instances, ﬂows have been observed as regions of slow mix-

135 oW o ing (i.e., Schlieren effect) that suggest inflowing water is 110 W o slightly denser than ambient lake water. All observed groundwater inflows are spatially separated from microbia- Fig. 1 Location of Pavilion and Kelly Lakes, British Columbia, Canada. lites by a minimum of about 10 m, and there are no obser- Modified from Forrest et al., 2010 and Lim et al., 2009. Image courtesy of vations of groundwater issuing directly from microbialites Pavilion Lake Research Project. or their chimneys.

0 Microbialites Several different morphologies of microbialites are found at 10 a variety of depths in Pavilion Lake (Laval et al., 2000; Lim et al., 2009, 2011). Microbialites range from several centimeters to meters in height and are found at depths ranging from 5 to 55 m (Fig. 3). Microbialites from shal- 20 lower depths (ca. 10–20 m) are porous and friable, while microbialites from intermediate depth (ca. 20–30 m) are more cohesive and generally larger. Microbialites at the 30 deepest depths (ca. 50 m) in the middle of the central

Depth (m) basin are very dense and have a black coating (Lim et al., 2009). The basins in which the microbialites are found are

40 characterized by low sedimentation rates (Lim et al., 2009). Pavilion Lake microbialites consist of mainly of mi-

2004 crite (fine-grained carbonate). The surfaces of the micro- 2005 bialites are coated in living microbial communities, with a 50 2006 biofilm approximately 5 mm thick, and fossil microbes are 2007 2008 often found in the immediate subsurface (Laval et al., 2009 2000; C. Harwood, unpubl. data). Microbialites have been 2010 60 estimated, based on several studies, to grow at a rate 0 10 20 between 0.025 and 0.05 mm year 1 (Laval et al., 2000; T (°C) Brady et al., 2009). Fig. 2 Temperature profiles for Pavilion Lake, BC for years 2004–2010. Surface waters reach roughly 20°C during summer months. Controls on microbialite precipitation Several small groundwater sources in various regions of Accretion of microbialites in Pavilion Lake is thought to Pavilion Lake have been observed by SCUBA divers. Direct be largely attributable to in situ carbonate precipitation, measurements show groundwater inflow temperature to be with little to no input coming from the trapping and bind- indistinguishable from the adjacent water column. In all ing of detrital carbonate, which is analogous to many

A C

Fig. 3 Microbialites of Pavilion and Kelly Lakes. (A) In situ photograph of microbialites currently found at 26 m water depth. Scale is 40 cm total. (B) Thin section of microbialite from 26 m water depth in crossed polars. Scale bar is 2 mm. (C) Image of Kelly Lake microbialite from 26 m water depth. Microbialite was originally found toppled over. Scale is 17 cm. Images courtesy of the Pavilion Lake Research Project. ancient forms (Laval et al., 2000). Pavilion Lake is super- cluded that the main source of carbon in the microbialites is saturated with respect to carbonate (Lim et al., 2009). lake water DIC derived from atmospheric CO2, although Traditionally, carbonate microbialites are thought to pre- there may be some radiogenically depleted groundwater cipitate though primary production, coupled with hetero- input. The shallow water microbialites (those at a depth of trophy, especially anaerobic bacterial sulfate reduction less than 20 m) are estimated to have a maximum of 9–13% (Visscher et al., 2000; Decho et al., 2005). Photosynthesis carbon input from groundwater. Microbialites located at takes up COs and locally increases alkalinity and initial pre- 20–26 m depth were found to possibly have slightly more cipitation in the organic matrix. Heterotrophy later carbon input from groundwater (estimated at 10–16%), degrades biomass, especially extrapolymeric substances while deep microbialites (those at depths greater than (EPS) which release bound calcium ions, furthering precip- 32 m) may have greater levels of carbon input from ground- itation (Decho et al., 2005). In Pavilion Lake, there is a water, although as mentioned above, the actual extent of component of aerobic heterotrophy, which might normally groundwater input is unknown (Brady et al., 2009). produce acid that would foster carbonate dissolution Elevated carbonate d13C values above expected abio- (Dupraz et al., 2009; Omelon et al., 2013). However, genic equilibrium values have been detected in microbia- Pavilion Lake is highly buffered (pH = 8.3; mean lites from depths of ~ 6–26 m and represent biosignatures 1 CaCO3 = 182 mg L ; Lim et al., 2009; Omelon et al., formed via a predominance of photosynthetic influences 2013) so dissolved inorganic carbon (DIC) inputs from on the isotopic geochemistry of the microenvironment the oxidation of EPS would not sufficiently budge the pH associated with the microbial community (Brady et al., and carbonate-precipitating capacity of the lake-wide sys- 2010; Brady et al., 2014; Russell et al., 2014). Interest- 13 tem. In short, despite the diverse microbial communities ingly, d Ccarb values from microbialites from depths living in the microbialites, the only metabolic systems below 26 m were generally within the range expected for expected to influence carbonate precipitation are photosyn- equilibrium precipitation (Brady et al., 2014; Russell thesis and heterotrophy. et al., 2014). Russell et al. (2014) characterized the Brady et al. (2009) measured the 14C content of phos- microbial and eukaryotic communities present in the mi- pholipid fatty acids (PLFA), DIC, and carbonate and con- crobialites and found that, contrary to what would be

© 2014 John Wiley & Sons Ltd Clumped isotope signatures of microbialites 57 expected, changes in community structure with depth do 1) A period of drought and higher summer temperatures not correspond to obvious changes in microbialite struc- prior to 6600 years before present; ture. However, distinctions in autotrophic and heterotro- 2) An end to the ‘warm’ period ~3500 ypb; and phic communities were observed between microbialites 3) Pulsed rises in lake level at 5600 and 2000 years before from <21 m and >26 m (Russell et al., 2014). The micro- present, associated with cooler waters (based on the pres- bial communities associated with the structures are in a ence of the aquatic gilled snail Valvata sincera helicoidea. stationary growth phase and have low turnover rates, although they are not nutrient limited (Schulze-Makuch KELLY LAKE et al., 2013). Kelly Lake is also situated in the Marble Range, 15 km west of Clinton in central British Columbia (Fig. 1), Age of Pavilion Lake microbialites although it is in a separate watershed from Pavilion Lake Shallow water microbialites (those found at 10–18 m water (Lim et al., 2011). Kelly Lake is dimictic and has a maxi- depth) are actively growing and thought to be precipitat- mum water depth of 40 m. Lake water is alkaline (pH ing calcium carbonate via photosynthetic activity, mainly ~8.3) and slightly supersaturated with respect to calcium during summer months (Brady et al., 2010; Brady et al., carbonate (Ferris et al., 1997; Lim et al., 2009). During 2014). The deeper microbialites may be actively growing summer, Kelly Lake thermally stratifies with a surface layer due to a combination of both photosynthesis and hetero- that is typically 17°C and a maximum thickness of 10 m trophy (Russell et al., 2014), although the present-day (Fig. 4). Below the surface layer, lake temperature profile of photosynthetically active radiation (PAR) indi- decreases rapid through a 2–3 m thick thermocline, below cates that the deepest structures (> 30 m) are receiving less which temperature gradually decreases to 4°C at the lake than 1% of surface radiation (see figure 4 in Brady et al., bottom, crossing 5°C at about 25 m depth. Surface water 2009), so the extent to which the deepest specimens are d13C values average 6.8& (V-PDB), but range from currently accreting is unclear. Radioisotope data have been 7.3 to 6.3& (V-PDB). Surface water samples collected reported in previous studies. Thus far, all of the microbia- by the PLRP during the 2006–2008 field seasons range in lites that have been measured are estimated to be younger d18O from 14.84 to 16.23& (V-SMOW) (Table 2). than the last glaciation (12 ka), although there is a range of apparent ages that have been reported (Laval et al., Microbialites 2000; Brady et al., 2009). A few microbialites at intermediate water depths (21.5 m) Significantly less work has been performed on Kelly Lake are found growing off of detrital tree branches. These microbialites compared with those found in Pavilion branches were dated using 14C (calibrated using the Int- Lake. Ferris et al. (1997) described shallowly submerged Cal04 database) and reported to be 1230 30 years before rocks and tree branches covered with well-lithified calcite present (Brady et al., 2009), which represents a maximum growth at depths of approximately 0.5 m. More recently, radiocarbon age for these particular microbialites. Additional D14 C measurements of microbialites growing on detrital 0 branches show evidence for accretion within the last 580 years (A. L. Brady, pers. comm. to V. A. Petryshyn and 5 A. K. Tripati). U–Th series dates were taken from microbialites found at intermediate depths (27–32 m) by Laval et al., 10 2000;. These results indicate that the structures started 15 growing ~6000 years before present. Brady et al. (2009) recovered an apparent age of 9550 30 years before pres- 20 ent for microbialites at 45 m water depth; however, the Depth (m) apparent age may be due to a relatively higher input of 14C- 25 depleted groundwater DIC at these deep depths and/or res- 30 ervoir effects. 2004 Significant changes in climate have occurred over the 2005 35 2006 interval recorded by our samples. Table 1 summarizes the 2007 2008 apparent age of microbialites found as a function of present- 40 day water depth. Constraints on past climate variability in 0 10 20 T (C) this region are limited, although the following trends have been reported based on previous work on lakes in British Fig. 4 Temperature profiles for Kelly Lake, BC for years 2004–2008. Sur- Columbia for the Holocene (Mathewes & King, 1989): face temperatures reach roughly 17°C during summer months.

Lim et al. (2009, 2011) reported large-scale (>1m) and three standard analyses were performed each day, and microbialites at up to 26 m depth in certain portions of every sample was analyzed in triplicate. Carbonate standards, the basin. A recent study shows that Kelly Lake micro- 25 C-equilibrated gases and ‘heated gases’ were analyzed. 13 bialites also have elevated d Ccarb values, although at Heated gases are composed of CO2 with a near-stochastic depths that differ from those identified in Pavilion Lake (0.0266&) distribution of isotopes between isotopologues. (S. Soles, A. Brady, G. Southam, D. Lim, & G. Slater, Gases with different bulk d18O and d13C ratios in quartz unpubl. data), Kelly Lake microbialites also show breakseals are heated to 1000°C for 2 h and then quenched evidence for relatively recent accretion as microbialites at room temperature. Standard gases are then purified and have been found growing on detrital logs that have been analyzed using the same protocol as sample gases. Typical 14C dated to ~ 210 years old (A. Brady, unpubl. results) precision is 0.005–0.020& (1 SE), equivalent to about 1–2°C, consistent with other studies (Eiler, 2007; Hunting- ton et al., 2009; Eagle et al., 2010; Tripati et al., 2010; METHODS Dennis et al., 2011; Thiagarajan et al., 2011). To monitor the presence of clean CO samples, we screened for the pres- Microbialite sampling 2 ence of contaminating molecules such as hydrocarbons and Microbialite samples were hand collected by the Pavilion sulfur compounds using mass 48 anomalies. A small subset Lake Research Project (PLRP) SCUBA divers and Deep- of samples were also measured at Caltech on two gas source worker submarines during the 2007, 2008, and 2009 field mass spectrometers using the same instrumental setup and seasons. Samples were collected from a range of depths, indicated that sample values are intercomparable between from 11.7 to 55 m in Pavilion Lake, and 11.7 to 22.7 m laboratories. Standard data are reported in Table S2. in Kelly Lake. Samples had outer surfaces (~3 cm) cut away to remove apparent biofilms from the carbonate to mini- Notation mize analytical interference from organic matter. These samples were rinsed in deionized water, allowed to dry Stable isotope data for samples are reported in Table 3 and overnight, and subsequently powdered with a mortar and in Table S1. 13C-18O bond order in carbon dioxide is pestle. Powdered samples were stored in a desiccator until described using the variable D47, which refers to the per the clumped isotope analysis was made. mil (&) deviation in the abundance of CO2 molecules with a mass of 47 amu from the abundance predicted by stochastic (random) mixing. Specifically D is defined as: Clumped isotope analysis 47 47 At least 40 mg of carbonate powder was taken from each mi- D ¼ R 47 2 crobialite to ensure a sufficient amount of CO2 for analysis 2R13 R18 þ 2R17 R18 þ R13 ðR17Þ in triplicate. Samples were measured on a specially modified 46 45 R R þ Thermo Fisher 253 gas source mass spectrometer dedicated 2 1 2R18 þ 2R13 R17 þðR17Þ R13 þ 2R17 to measuring clumped isotopes in CO . The mass spectrom- 2 1000 eter has been configured for the analysis of multiply substituted isotopologues of CO2 and is attached to a custom- built automated system for carbonate sample preparation, where R refers to the ratio of the minor isotopologue to the purification, and introduction (Passey et al., 2010). They major isotopologue of the molecule (or atom) of interest. system is composed of (i) a Costech Zero Blank autosampler As sample reactions were carried out at 90°C, we applied a made of stainless steel that will pull high vacuum, (ii) a com- published (Henkes et al., 2013) empirically derived acid & D mon acid bath held at 90°C for phosphoric acid digestion of digestion fractionation correction of 0.092 for 47 values samples, (iii) cryogenic traps (dry ice and ethanol, and liquid to facilitate comparison to the published calcite line calibra- ° nitrogen) for the purification and collection of CO2 and tion in which samples were reacted at 25 C (Ghosh et al., D removal of water and other gases with low vapor pressures, 2006). Errors in reported 47 values and calculated temper- (iv) a gas chromatograph with a packed column and a cryo- atures include the propagated uncertainty in heated gas genic trap to further purify CO2 through the removal of determination and in sample measurement (Huntington organic contaminants, with helium being used as a carrier et al., 2009). Data are reported on the ‘absolute reference gas, (v) cryogenic traps to remove the helium, and (vi) a final frame’ (Dennis et al., 2011) proposed for clumped isotope set of valves and traps to purify CO2 and transfer it into the studies of CO2 based on the analysis of equilibrated CO2 bellows of the mass spectrometer. Measurements are made gases and carbonate standards, with data used for calcula- 18 to yield a stable 16-volt signal for mass 44, with peak center- tions in Tables S1–S3. For calcite d O calculations an acid ing, background measurement, and pressure balancing digestion fractionation factor of 1.00821000 was used before each acquisition. Approximately two sample analyses (Swart et al., 1991).

Table 3 Stable isotope ratios for individual analyses of modern samples from Pavilion and Kelly Lakes reported in this study, and calculated averages. Calcite oxygen and carbon isotope values are reported relative to V-PDB standard, and clumped isotope values are reported on absolute reference frame (ARF)

13 18 Sample ID Depth (m) Depth (ft) d C(&)SD d O calcite (&)SD D47 SE Mean D47 Error

Pavilion Lake TP 062207 PL 35 FT 11.7 35 0.692 0.012 11.482 0.008 0.739 0.014 0.732 0.008 0.673 0.002 11.281 0.006 0.741 0.013 0.649 0.002 11.478 0.005 0.715 0.012 TP 180711-0:65 PL 65 ft 21.7 65 0.956 0.002 10.424 0.008 0.740 0.013 0.748 0.005 0.986 0.003 10.366 0.005 0.747 0.011 0.986 0.006 10.541 0.007 0.757 0.007 TP1707 11-81APL85 85 ft 28.3 85 0.465 0.003 10.746 0.005 0.742 0.012 0.741 0.001 0.446 0.002 10.682 0.005 0.740 0.016 0.403 0.006 10.468 0.018 0.742 0.026 2000806-28D-PV-55m 55 165 0.304 0.004 10.027 0.007 0.745 0.013 0.757 0.013 0.341 0.002 9.902 0.003 0.782 0.011 0.343 0.002 9.902 0.003 0.744 0.011 Kelly Lake KC 220711-35D 35 FEET 11.7 35 2.444 0.002 16.666 0.010 0.720 0.014 0.731 0.007 2.402 0.003 16.632 0.015 0.729 0.010 2.468 0.001 16.681 0.006 0.744 0.012 KL 2007 11-65B 68 feet 22.7 68 1.043 0.010 12.608 0.014 0.766 0.008 0.759 0.010 1.148 0.002 12.617 0.008 0.752 0.009

Calculation of temperatures and water isotope values values with depth in Pavilion Lake, although all were still within the expected abiogenic range of precipitation values Temperatures are calculated from clumped isotope data on (Brady et al., 2014). In Kelly Lake, samples retrieved from the absolute reference frame by applying the equation of the shallowest site yielded a d13Cof2.4& V-PDB, while Ghosh et al. (2006) as reported in Dennis et al. (2011). deeper samples had a d13C value of 1.1& V-PDB. A This equation is: recent study in Kelly Lake also shows higher d13C values at 6 2 deeper depths, with samples at 26 m having enriched val- D47 ¼ð0:0636 0:0049 10 Þ=T ð0:0047 0:0520Þ ues above predicted equilibrium (Soles et al., pers. where T is temperature in Kelvin. Other published D comm.). 47 d18 & calibrations were also used to estimate microbialite Calcite O signatures ranged from 11.4 to 9.9 & precipitation temperature (Dennis et al., 2011; Eagle et al., V-PDB in Pavilion Lake and 16.7 to 12.6 V-PDB in d18 2013a; Zaarur et al., 2013) to explore the sensitivity of our Kelly Lake (Fig. 5). Table 3 shows how microbialite O calculations to choice of calibration. Temperatures estimated in both lakes does systematically increase, with a change of & & using sample D values were used to calculate the d18Oof about 1.5 in Pavilion Lake and about 4.1 in Kelly 47 d18 water from which the mineral precipitated using paired mea- Lake. Water O at both lakes does exhibit some variabil- d18 surements of carbonate d18O and the following published ity (Table 2). We calculate equilibrium calcite O values equation from Kim & O’Neil (1997): for calcite formed under modern conditions in both lakes at the water depths our samples were collected from 1000 ln a ¼ð18:03 103Þ=T 32:42 (Table 4). At both lakes, the shallowest samples yield cal- calciteH2O cite d18O values that are similar to predicted equilibrium 18 where T is in Kelvin. d O values. At Pavilion Lake, all of the other samples (i.e., those collected below the thermocline) have lower d18O values than expected for precipitation under present- RESULTS day conditions (Table 4): Specifically measured values are about 1.5& too low to be consistent with equilibrium cal- Stable isotopes cite growth under present-day conditions. At Kelly Lake, In our dataset, samples show little variation in d13C with the deeper sample has a calcite d18O value that is only depth, ranging from 0.3 to + 0.96& V-PDB (Table 3; slightly lower than expected for equilibrium growth under Fig. 5). Microbialites found at 55 m depth in Pavilion modern conditions. Lake consistently had the lightest d13C signatures, This pattern with depth could reflect equilibrium calcite although generally there are no other clear trends with precipitation in the modern microbialites and disequilibrium depth in this lake (Table 3). These findings are consistent growth in only the older microbialites at Pavilion Lake. For with previous reports showing a slight decline in d13C example, elevated pH within microbial biomass has the

Age increasing Age increasing?

Modern Early Holocene Modern Early Holocene? Pavilion Lake Kelly Lake —8 —10 (‰) —9 (‰) —12 V-PDB —10 V-PDB O O —14 18 18 δ —11 δ

—12 —16 Calcite —13 Calcite —18 0204060 0 5 10 15 20 25 Depth (m) Depth (m) 1.5 0 (‰) (‰) 1.0 —1 V-PDB V-PDB C 0.5 C 13 13 δ δ —2 0.0 Fig. 5 Microbialite d18O and d13C in Pavilion Calcite Calcite —0.5 —3 and Kelly Lake plotted as a function of water 0204060 0 5 10 15 20 25 depth. Also shown are predicted equilibrium Depth (m) Depth (m) d18O values given present-day hydrographic Measured in microbialites conditions. Panels on left are for Pavilion Predicted equilibrium values for present-day conditions Lake, and panels on right are for Kelly Lake.

Table 4 Predicted calcite oxygen isotope values compared to measured values in & relative to V-PDB standard. Current summer water temperatures used for calculations

Sample ID Depth (m) Water T (°C) Max. pred. d18O Min. pred. d18O Measured d18OSE

Pavilion Lake TP 062207 PL 35 FT 11.7 19 11.8 12.5 11.41 0.07 TP 180711-0:65 PL 65 ft 21.7 6 9.0 9.7 10.44 0.05 TP1707 11-81APL85 85 ft 28.3 4 8.5 9.2 10.63 0.08 2000806-28D-PV-DM-55m 55 4 8.5 9.2 9.94 0.04 Kelly Lake KC 220711-35D 35 FEET 11.7 17 15.5 16.9 16.66 0.01 KL 2007 11-65B 68 feet 22.7 4 12.7 14.1 12.61 0.01 potential to impact the d18O of the DIC pool, which in turn Clumped isotope temperatures could be reﬂected in calcite d18O (Brady et al., 2010). Alter- At Pavilion Lake, samples retrieved from shallow depths natively, the trends in d18O with water depth could arise (11.7 m) yielded an average Δ -based temperature of from changes in paleotemperature and/or paleohydrology 47 20.7 1.7°C (Table 5; Fig. 6) when calculated using the associated with lake level ﬂuctuations that are archived in the equation of Ghosh et al. (2006) (on the absolute reference equilibrium calcite d18O values of all of the microbialites. frame after Dennis et al., 2011). Intermediate samples from

Table 5 Reconstructed temperatures for Pavilion and Kelly Lake calculated using different temperature calibrations compared to current summer water temperatures

Sample ID Depth (m) Water T (°C) TD47 (°C) Cal. 1 SE TD47 (°C) Cal. 2 SE TD47 (°C) Cal. 3 SE TD47 (°C) Cal. 4 SE

Pavilion Lake TP 062207 PL 35 FT 11.7 19 20.7 1.7 17.7 1.9 12.9 2.9 18.2 1.9 TP 180711-0:65 PL 65 ft 21.7 6 17.5 1.0 14.2 1.0 7.4 1.6 14.7 1.1 TP1707 11-81APL85 85 ft 28.3 4 18.8 0.1 15.6 0.1 9.6 0.2 16.1 0.1 2000806-28D-PV-DM-55m 55 4 15.8 2.3 12.3 2.6 4.5 3.9 12.7 2.6 Kelly Lake KC 220711-35D 35 FEET 11.7 17 20.9 1.4 17.8 1.5 13.1 2.4 18.4 1.6 KL 2007 11-65B 68 feet 22.7 4 15.4 1.9 11.9 2.0 3.9 3.1 12.3 2.1

Cal 1 = Ghosh et al. (2006). Cal 2 = Eagle et al. (2013a) (all biogenics calibration). Cal 3 = Dennis et al. (2011). Cal 4 = Zaarur et al. (2013).

Age increasing Age increasing?

Modern Early Holocene Modern Early Holocene? Pavilion Lake Kelly Lake 30 30

20 20 C) C) ° ° ( ( 47 47 T T 10 10

0 0 0204060 0 5 10 15 20 25 Depth (m) Depth (m)

) —7 —10 ) ‰ ‰ ( —8 ( —12 —9 V-SMOW V-SMOW —14 O O

18 —10 18 δ δ —11 —16 Water —12 Water —18 Fig. 6 Reconstructed temperature and water 0204060 0 5 10 15 20 25 d18O values based on clumped isotope data. Depth (m) Depth (m) Also shown are present-day hydrographic conditions. Open symbol corresponds to Reconstructed values possible outlier. Modern hydrographic data below the thermocline (those from a depth of 21.7 m) yield is that microbialites are precipitating carbonate in appar- 18 an average Δ47-temperature of 17.5 1.0°C. Samples from ent equilibrium and that both carbonate d O and Δ47 28.3 m (also below the thermocline) have Δ47 signatures are likely tracking climatically induced changes in lake that yield growth temperatures of 18.8 0.1°C. Finally, level. the deepest samples (the ‘deep mounds’ found at 55 m depth) were found to form at 15.8 2.3°C, although one Holocene climate/Lake level change analysis of the three replicates was measured with a Δ47 value that was higher than the rest of the population, correspond- The Pavilion Lake microbialites show evidence for active ing to a temperature of 11.2°C (open symbol in Fig. 6). If photosynthesis and accretion, in particular during the sum- this outlier is excluded, and the outer replicates are averaged, mer months (Brady et al., 2010; Brady et al., 2014) over then calculated temperatures are 18.2°C. At Kelly Lake, the last 1200 years (Brady et al., 2009). In contrast, the shallow water microbialites (11.7 m) yielded an average Δ47 deepest (55 m) microbialites appear to be signiﬁcantly –temperature of 20.9 1.4°C. Samples from 22.7 m have older than shallower microbialites by several thousand

Δ47 –based formation temperatures of 15.4 1.9°C. Similar years, although lake reservoir effects must be considered temperatures are calculated using the calibrations of Eagle when estimating radiocarbon ages (Laval et al., 2000; et al., 2013a (for all biogenic materials) and Zaarur et al., Brady et al., 2009). Mathewes & King (1989) have per- 2013. Carbonate precipitation temperatures estimated using formed palynologic studies on several lakes from the region several different published calibrations are shown in Table 5. for the Holocene. They concluded that British Columbia went through a dry period prior to 6600 years before present, which caused low lake levels and resulted in basins DISCUSSION that were susceptible to fluctuations given a relative small In both lakes, the shallow water microbialites match the change in net precipitation, and speculated there may have current temperature profile, although the deeper water been drought as well as warmer temperatures in the sum- samples do not. Several things could account for this: (i) mer months (Mathewes & King, 1989). Their study found changes in lake level and climate over the Holocene, (ii) an abrupt change to cooler and wetter climates approxi- contamination by organic matter, (iii) post-precipitation mately 5660 years before present, with a transition to con- diagenesis of the microbialites skewing the clumped iso- ditions similar to at present occurring about 2000 years tope signature, (iv) an influx of warm groundwater induc- before present. This study, when taken in combination ing carbonate precipitation, and (v) kinetic isotope effects. with radioisotope work, implies that the deepest microbia- Below, we discuss each of these possible interpretations in lites may have started to form during a period of drought, detail. We conclude the most parsimonious interpretation when lake levels would have been significantly lower.

Is it reasonable to assume that the microbialites that are values, consistent with climatically induced lake level fluc- today found at depth actually would have grown near the tuations. surface? Multiple studies (Omelon et al., 2013; Schulze- The deepest microbialites in Pavilion Lake yield a water Makuch et al., 2013; Russell et al., 2014) show a close d18Oof9.5 0.5& (V-SMOW); this value drops to association between photosynthetic bacteria and present- 8.8 0.5& if data for one replicate that may be an out- day microbialite growth in Pavilion Lake. The microbialites lier are excluded (outlier is shown as open symbol in are actively forming at shallow and intermediate depths, as Fig. 6). These values are about 1.6 to 2.3& higher than the calcification of filaments can readily be seen. The the present average lake value. Similarly, high water d18O results of this study indicate that the optimal zone for mi- values are reconstructed for all of the older microbialites at crobialite formation is within the surface mixing zone, Pavilion Lake. where summer temperatures stay around 19°C. During the In Kelly Lake, microbialites currently found at 11.7 m mid-Holocene, when the deeper mounds are dated as hav- depth yield an average water d18Oof15.1 0.3& ing begun to form, arid conditions in the region may have (V-SMOW) (Table 6), which closely matches the measured lowered lake levels enough to favor the activity of photo- average summer value (Table 2). However, the deeper mi- synthetic bacteria and the precipitation of carbonate. As cli- crobialite supports a substantially higher past water d18O mate shifted to wetter times, lake levels began to rise, and of 12.2 0.4& (Table 6). This microbialite has not microbialite growth migrated upward. The microbialites been dated. are currently accreting, so there should be a pull toward The reconstructed variations in water d18O in both lakes modern temperatures. However, during sample prepara- differ in magnitude (Fig. 6). A 2.9& difference is calcu- tion, the exterior of the microbialites was removed to elim- lated at Kelly Lake, while fluctuations of 0.3 to 0.4& are inate organic material, perhaps accounting for the lack of calculated at Pavilion Lake. This difference is possibly due ‘modern’ recorded temperatures. to lake size: A larger lake is more buffered against fluctua- One way to validate the hypothesis that changes in tions in water d18O. Alternatively, it is possible that the Holocene lake levels drove the initiation of microbialite deeper microbialite from Kelly Lake is recording climate precipitation in these two basins is to examine the d18Oof fluctuations at a different time. the water of formation. While this has in the past been These results are consistent with growth of all of the impossible to quantify (and therefore has had to be microbialites in the mixed layer and photic zone, with assumed), we were able to determine fluid d18O by com- microbialites tracking changes in lake level. The deepest, bining clumped isotope-derived temperatures with carbon- older microbialites that were measured in this study may ate d18O measurements (Fig. 6). Calculations in Table 6 then have formed during periods of enhanced evaporation show the sensitivity of reconstructed water d18O values to relative to precipitation, and lower lake level, when lake 18 choice of D47-temperature calibration. water was isotopically more enriched in O. If both Pavilion and Kelly Lakes were much shallower in the past due to changes in the balance of precipitation and Contamination evaporation, and/or due to changes in vapor transport or watershed recycling, it should be reflected in the d18Oof No evidence for contamination was found that could lake water. For example, evaporation that accompanies a account for the temperatures we observed. In some cases, period of drought should promote an increase in the d18O the gas chromatograph and purification procedure can fail signature of the water of formation. In both lakes, deeper to remove organic contaminants from the system, which in and presumed older microbialites yield higher water d18O turn can result in isobars at masses 44–49. However, the

Table 6 Reconstructed oxygen isotope values for waters (V-SMOW, &) associated with microbialite precipitation based on clumped isotope temperatures and calcite–water fractionation factors from Kim & O’Neil (1997). Modern values for Pavilion Lake range from 10.7 to 11.4& and for Kelly Lake range from 14.8 to 16.6&. Values calculated using different calibrations

Sample ID Depth (feet) d18Ow Cal. 1 SE d18Ow Cal. 2 SE d18Ow Cal. 3 SE d18Ow Cal. 4 SE

Pavilion Lake TP 062207 PL 35 FT 11.7 9.9 0.3 10.5 0.4 11.6 0.6 10.4 0.4 TP 180711-0:65 PL 65 ft 21.7 9.6 0.2 10.3 0.3 11.8 0.4 10.2 0.3 TP1707 11-81APL85 85 ft 28.3 9.5 0.1 10.2 0.1 11.5 0.1 10.1 0.1 2000806-28D-PV-DM-55m 55 9.5 0.5 10.2 0.5 12.0 0.9 10.1 0.6 Kelly Lake KC 220711-35D 35 FEET 11.7 15.1 0.3 15.8 0.3 16.8 0.5 15.7 0.3 KL 2007 11-65B 68 feet 22.7 12.2 0.4 13.0 0.4 14.8 0.7 12.9 0.4

Cal. 1 = Ghosh et al. (2006). Cal. 2 = Eagle et al. (2013a) (all biogenics calibration). Cal. 3 = Dennis et al. (2011). Cal. 4 = Zaarur et al. (2013).

© 2014 John Wiley & Sons Ltd Clumped isotope signatures of microbialites 63 presence of these contaminants can be discovered by simul- small amount of temperate water could overwhelm the taneously collecting data on masses 47, 48, and 49. The mass of 6°C water below the thermocline and change the presence of organics will cause not only an excess of mass temperature enough to skew the clumped isotope results. 47, but also an excess of mass 48 and 49 (Eiler & Schau- ble, 2004; Tripati et al., 2010). Comparison with mass Equilibrium precipitation or kinetic isotope effects? 48–49 data for clean standard gases and carbonate standards can be used to screen for contamination, as can high Most modern carbonates surveyed to date generally adhere internal errors within a single measurement, and poor to the inorganic calcite calibration reported by Ghosh et al. reproducibility. In this case, mass 48 and 49 yielded no (2006), including both abiogenic precipitates (Passey et al., anomalies that would be consistent with high degrees of 2010; Eagle et al., 2013b) and biogenic precipitates (Came organic contamination (Table S1; Fig. S1). et al., 2007; Tripati et al., 2010; Thiagarajan et al., 2011; Eagle et al., 2013a,b). However, deviations from the experimentally calibrated D temperature equilibrium Diagenesis 47 curve of Ghosh et al. (2006) have been reported (Ghosh The lacustrine microbialites have not been buried or et al., 2006; Affek et al., 2008; Tripati et al., 2010; heated in any significant way. Both lakes are saturated with Da€eron et al., 2011; Saenger et al., 2012; Eagle et al., respect to calcium carbonate, so dissolution and reprecipi- 2013a). To date, modern/recent lacustrine carbonates tation is unlikely. Although secondary precipitation may have only been reported in one study (Huntington et al., have occurred, and there is some suggestion of infilling of 2010) and are for micrites and aquatic mollusks. The the microbialite framework, particular at deeper depths reported D47 values are consistent with carbonate precipita- (>45 m) (Omelon et al., 2013), petrographic studies reveal tion in mid-latitude lakes during the spring and summer no obvious secondary cement phases. The structures are months. The application of the shallower calibration slope uniformly micritic, with no evidence of aggrading neomor- reported by Eagle et al. (2013a) yielded slightly higher phism that would indicate recrystallization. In fact, if there temperatures (on the order of 1–2°C), but did not signifi- was significant secondary precipitation over time, the tem- cantly affect the results of this study. While there are diver- peratures recovered from the D47 measurements of micro- gences in different calibrations at low and high bialites below the thermocline should be closer to the temperatures, at the range of temperatures presented in current lake bottom temperature of 6°C, instead of yield- this study, the choice of calibration does not change our ing higher temperatures similar to surface waters. Given conclusions. Below, we explore several situations that the history and the petrography of the samples, it is unli- would cause the Pavilion and Kelly Lake microbialites to kely that the observed signal is due to diagenetic processes. precipitate out of thermodynamic equilibrium with the ambient lake water. Hydrothermal groundwater input Equilibrium precipitation? There is no evidence for hydrothermal input that would One possibility is that all of the data from microbialites are skew the clumped isotope temperature. As noted in the accurately recording water temperatures and are equilib- introduction, Pavilion Lake is a groundwater-fed system. rium precipitates. If true, then most or all of the carbonate Inflows into Kelly Lake are less well constrained. Brady found in the deeper water microbialites grew near the et al. (2009) performed some stable isotope analyses on water surface (i.e., at shallower water depths where condi- local and regional groundwater and suggested that a com- tions may be more optimal for microbialite growth) during ponent of the microbialite carbon may be groundwater- a period when lake level was lower. In that scenario, the related. However, the local groundwater has a d13C signa- deeper water microbialites (those at depths >26 m) would ture (9.9& VPDB) what is much lighter than that of the be older. Radiocarbon and U-series dates support this microbialites (which average +1.1 to 1.0& VPDB in interpretation (Laval et al., 2000; Brady et al., 2009.). Pavilion Lake and 1.1 to 2.5& VPDB in Kelly Lake), Further, the reconstructed water d18O values for the youn- indicating that the microbialites did not source their carbon gest microbialites in both lakes are similar to measured entirely from the local groundwater, but largely from the water values, consistent with carbonate precipitation in 18 lake DIC. It is possible that groundwater influenced micro- near equilibrium for both D47 and d O. bialite formation; however, Brady et al. (2009) conclude that groundwater carbon is not the main source of carbon The possibility of CO2 degassing to the microbialites. Additionally, the local and regional The CO2 degassing is not a process which is likely relevant groundwater is not hydrothermal in nature. Schulze- to carbonate precipitation in microbialites; however, we do Makuch et al. (2013) measured several well, all which ran- explore the systematics and how it would affect the appar- ged from 10–13°C. It is unlikely that the introduction of a ent temperatures. It has been shown that structures that

precipitate rapidly from the degassing of CO2 (such as fast- ones express a kinetic isotope effect associated with carbon- drip speleothems) are out of isotopic equilibrium and do ate growth at low temperatures. Although we cannot not adhere to the published calibration curves (Affek et al., exclude this possibility, we deem the latter to be somewhat 2008). These out-of-equilibrium materials show an enrich- less probable as the kinetic isotope effect would have to 18 ment in d O and a corresponding decrease in D47 (which fortuitously produce D47 signatures that result in apparent would artiﬁcially increase the apparent temperature). Guo temperatures similar to those of surface microbialites. et al. (2009) predicts a D47 decrease of 0.017& (roughly Solution pH and CO2 hydrolysis would, in principle, 3°C) for every 1& increase in d18Oat25°C. Further, the affect both the clumped isotope composition and d18Oof data for both lakes exhibit an ~0.03& decrease in D47 for the DIC pool in a systematic manner, which can be probed 18 every 1& decrease in carbonate d O. by examining whether there is a correlation between D47 and d18O and by comparing the slope of a regression to

Vital effects in D47 associated with photoautotrophy experimental or theoretical predictions (A. Tripati, unpubl. While evidence for photosynthetic influences on carbonate data). To explore whether solution chemistry might be precipitation, observed as enrichments in 13C above equi- important, we compare our data to experimental results librium, has been seen in Pavilion Lake, such enrichments (A. Tripati, unpubl. data). Figure 7 is a cross-plot of appar- are generally present in microbialites from depths of ~ 11 ent disequilibrium in d18O compared to apparent disequi- 18 to 20 m (Brady et al., 2010; Brady et al., 2014). Micro- librium in D47. Apparent disequilibrium in d Ois 13 bialites from below 26 m have d Ccarb values that are gen- calculated by taking the difference between the measured erally within the range expected for precipitation in calcite d18O and predicted calcite d18O (assuming precipi- equilibrium with the ambient lake water (Brady et al., tation at equilibrium in waters identical in temperature and 2014; Russell et al., 2014). Vital effects associated with water d18O to the lake). Similarly, apparent disequilibrium precipitation within the microbial biofilms could be in D47 is calculated by taking the difference between the expected to preserve downcore within the structures even measured D47 and the predicted equilibrium value for pre- after removal of the organic matter covering the microbia- cipitation in waters corresponding to the modern tempera- lites. Calcite d18O values do not vary systematically with ture profile (Table S4). It is clear that the trends observed depth in Pavilion Lake (~1.5&) and vary slightly more in in the two lakes differ from each other in direction. The 18 our sparse dataset for Kelly Lake (~4&), as discussed observed D47-d O slope of a regression through the data above; a more detailed dataset on calcite d18O values from for Pavilion Lake is 0.029 0.001 and in Kelly Lake is biofilm-associated carbonates show inconsistent variation 0.035 0.000. The slope for Pavilion Lake is substan- with depth (Brady et al., 2014). The lack of a systematic tially greater in magnitude than what is expected for a pH pattern with water depth is inconsistent with vital effects in effect, and the data for Kelly Lake are of the wrong magni-

D47 due to photoautotrophy. tude and sign (A. Tripati, unpubl. data). Growth at low 13 The only microbialites recording a d Ccarb signature temperatures, where the kinetics of CO2 hydrolysis is that is out of equilibrium with the lake water are those slower, would induce an apparent bias to higher D47 values dominated by photoautotrophy that are forming ~11 to 20 m water depth. It is possible that photosynthesis causes a ‘vital effect’. However, such an effect has not been 0.02 (‰) Pavilion Lake reported in coccoliths (Tripati et al., 2010). Further, sur- 47 0.00 Kelly Lake face microbialites record a temperature within error of summer averages. —0.02 If the vital effect of photoautotrophy was responsible for artiﬁcially increasing all the apparent temperatures, we —0.04 would also expect to see an increase in the percent (or —0.06 activity) of photoautotrophy with depth to explain the clumped isotope data. However, the opposite trend is —0.08 observed: Photoautotrophy decreases with depth, and heterotrophy increases (as does the percent of eukaryotic —0.10 Observed - modern equil. Δ equil. modern - Observed algae). —3 —2 —1 0 1 2 Observed - modern equil. calcite δ18O (‰) Kinetic effect associated with solution pH, growth rate, d18 and/or temperature expressed in deeper microbialites Fig. 7 Comparison of apparent disequilibrium in O compared to apparent disequilibrium in D47. Apparent disequilibrium is calculated by taking Another possibility is that all of the microbialites record the difference between the measured calcite values and predicted values kinetic effects, or the shallow water microbialites in both assuming precipitation at equilibrium in waters corresponding to the mod- lakes record equilibrium precipitation, while the deeper ern hydrographic proﬁle of both lakes.

© 2014 John Wiley & Sons Ltd Clumped isotope signatures of microbialites 65 and colder temperatures, the opposite of what is observed. CONCLUSIONS Growth rates may also be slower at depth, which would tend to favor equilibrium precipitation if all other factors Pavilion and Kelly Lake microbialites presented an excellent are equal. test case for the use of the carbonate clumped isotope paleothermometer in the reconstruction of temperatures and Unidentiﬁed precipitation mechanisms, chemical limnology, given the availability of water temperature and stratiﬁcation, or microbial community changes d18O data, and the radiometric dates available for some of

Another possibility is if there were completely different the samples. The D47 data from modern samples are precipitation mechanisms responsible for the formation of consistent with the greatest amounts of carbonate precipi- the deep water microbialites, associated with either large tation by the microbialite communities occurring in the changes in lake chemistry or microbial community below mixing zone during warm months (such as June, July, or 20 m water depth. Lake chemistry is fairly consistent August), consistent with other types of mid-latitude terres- through the water column, although the change in the cal- trial and marine carbonate precipitates. cium carbonate saturation index does increase slightly with Clumped isotope-derived estimates of water d18O can be depth (Lim et al., 2009). Microbial community structure simply interpreted to reflect fluctuations in hydrology that does change below 26 m, with a higher percentage of are linked to variations in lake level. Reconstructed water photoautotrophs found at shallow depths and more photo- d18O values are consistent with reports of more arid condi- heterotrophs found in deeper structures (Russell et al., tions during the mid-Holocene, when lake levels in the 2014). However, there is no apparent correspondence region were lower. We suggest the bulk of the older between microbial community and microbialite structure microbialites may have grown during times of drought, (see Fig. 6, Russell et al., 2014), and in fact, there is a while the shallower/younger microbialites grew in the cur- high degree of similarity between the communities of deep rent climate regime. Given the results of this study, and shallow water forms. Without a large and noticeable clumped isotope and oxygen isotope analyses of microbia- shift in community structure, and in light of the radiomet- lites may be a promising tool for the reconstruction of ric ages, it is difficult to pin the apparent temperatures on water temperatures and lake history in analogous planetary changes in metabolic process with depth. environments.

Microbialite formation associated with climatic ACKNOWLEDGMENTS ﬂuctuations We wish to thank all those on the Pavilion Lake Research D d18 Given the evidence for equilibrium 47 and O values in Project, especially Cara Harwood and Jennifer Biddle, modern microbialites in these lakes, and the arguments Frank Corsetti for the use of his microscope, and the Tri- above, we suggest the simplest explanation for the trends pati Lab research group. This work was supported by NSF D with age (relative stability in 47 values and reconstructed grants EAR-1352212, EAR-1325054, OCE-1437166, d18 temperatures, and variations in carbonate O and water EAR-0949191 and ARC-1215551, ACS grant 51182- d18 O) is that water isotopes have changed over the Holo- DNI2, DOE grant DE-FG02-13ER16402, and a Hellman cene associated with climate, explaining the observed Fellowship. This is PLRP publication #13-04. radiometric ages. It is plausible that the structures prefer to grow in the mixing zone, where light availability favors the growth of photosynthetic communities that build micro- REFERENCES bialites in these lakes. The deepest microbialites may have Affek HP, Bar-Matthews M, Ayalon A, Matthews A, Eiler JM begun growing prior to 6600 ypb, when the lake was (2008) Glacial/interglacial temperature variations in Soreq cave much smaller. Over the Holocene, as net evaporation was speleothems as recorded by ‘clumped isotope’ thermometry. reduced, lake waters began to rise and waters became less Geochimica et Cosmochimica Acta 72, 5351–5360. enriched in 18O, as recorded in the chemistry of the micro- Allwood AC, Walter MR, Kamber BS, Marshall CP, Burch IW (2006) Stromatolite reef from the Early Archaean era of bialites. As a wetter climate returned, the zone of micobia- Australia. Nature 441, 714–718. lite precipitation migrated upward. As long as there is Batchelor MT, Burne RV, Henry BI, Watt SD (2000) some light availability, active photosynthesis and accretion Deterministic KPZ model for stromatolite laminae. Physica Acta continued on all the microbialite surfaces, but the domi- 282, 123–136. nant episode of accretion occurred when the microbialites Brady AL, Slater G, Laval B, Lim DSS (2009) Constraining carbon sources and growth rates of freshwater microbialites in were situated in the thermocline. Simulations of water 14 Pavilion Lake using C analysis. Geobiology 7, 544–555. d18 O in an isotope-enabled model may resolve whether Brady AL, Slater GF, Omelon CR, Southam G, Druschel G, water d18O and lake level ﬂuctuations are in fact linked in Andersen DT, Hawes I, Laval B, Lim DSS (2010) the region during the Holocene. Photosynthetic isotope biosignatures in laminated micro-

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