Oxygenated Mesoproterozoic lake revealed throughmagnetic mineralogySarah P. Slotznicka,1, Nicholas L. Swanson-Hysella, and Erik A. Sperlingb

aDepartment of Earth and Planetary Science, University of California, Berkeley, CA 94720; and bDepartment of Geological Sciences, Stanford University,Stanford, CA 94305

Edited by Paul F. Hoffman, University of Victoria, Victoria, BC, Canada, and approved October 29, 2018 (received for review August 4, 2018)

Terrestrial environments have been suggested as an oxic havenfor eukaryotic life and diversification during portions of the Pro-terozoic Eon when the ocean was dominantly anoxic. However,iron speciation and Fe/Al data from the ca. 1.1-billion-year-oldNonesuch Formation, deposited in a large lake and bearing adiverse assemblage of early eukaryotes, are interpreted to indi-cate persistently anoxic conditions. To shed light on these distincthypotheses, we analyzed two drill cores spanning the trans-gression into the lake and its subsequent shallowing. While theproportion of highly reactive to total iron (FeHR/FeT) is consis-tent through the sediments and typically in the range takento be equivocal between anoxic and oxic conditions, magneticexperiments and petrographic data reveal that iron exists inthree distinct mineral assemblages resulting from an oxycline. Inthe deepest waters, reductive dissolution of iron oxides recordsan anoxic environment. However, the remainder of the sedi-mentary succession has iron oxide assemblages indicative of anoxygenated environment. At intermediate water depths, a mixed-phase facies with hematite and magnetite indicates low oxygenconditions. In the shallowest waters of the lake, nearly every ironoxide has been oxidized to its most oxidized form, hematite. Com-bining magnetics and textural analyses results in a more nuancedunderstanding of ambiguous geochemical signals and indicatesthat for much of its temporal duration, and throughout much ofits water column, there was oxygen in the waters of PaleolakeNonesuch.

Proterozoic | oxygen | iron speciation | lacustrine environments |eukaryotic evolution

Following the origin of eukaryotic life in the PaleoproterozoicEra (2,500–1,600 Ma), eukaryotic diversity is interpreted to

have remained relatively low in marine environments through-out the Mesoproterozoic Era (1,600–1,000 Ma) until ca. 800 Maduring the Neoproterozoic Era (1, 2). A hypothesis to explaindelayed eukaryotic diversification is that marine environmentsin a relatively low-oxygen world were prone to the upwellingof anoxic, and sometimes sulfidic, waters from widespread oxy-gen minimum zones (refs. 1, 3–5, but see ref. 6). The inhibitoryeffect of low-oxygen waters on aerobic eukaryotic life holds truewhether hypoxic conditions were caused by low atmosphericoxygen—as commonly assumed—or by shallow remineraliza-tion of sinking organic matter (7). This potential challenge foreukaryotic life in the marine realm has led to the suggestion thatoxygenated terrestrial environments may have been cradles ofeukaryotic diversification (4, 8).

Microfossils recovered from the Torridonian sequence ofScotland and the Nonesuch Formation of North America havebeen interpreted to indicate that by ca. 1.1 billion years agofreshwater habitats were colonized by eukaryotes as well ascyanobacteria (4, 8, 13). Recovered specimens from the None-such Formation include Valeria lophostriata (4), which is con-sidered to be diagnostically eukaryotic as the complex wallmorphologies and microstructures could not be generated byan organism that does not have a cytoskeleton and endomem-brane system (14). The microfossil record of the Nonesuch

Formation has been further interpreted to indicate the pres-ence of more than 50 different species (4). This record isargued to be more diverse than similar-aged marine assemblages,which leads to the interpretation that lacustrine environmentswith stable oxygenated waters may have been more hospitableto eukaryotic evolution than marine ones (4). Early oxygena-tion of lacustrine environments during the Mesoproterozoichas also been proposed based on large sulfur isotope frac-tionations from sedimentary rocks of the Stoer and Torridongroups that were interpreted to have resulted from oxidativesulfur cycling (15). However, this interpretation is equivocalgiven that such fractionation can arise without oxidative cycling(16, 17).

The chemistry and mineralogy of iron and oxygen in theenvironment are tightly interwoven, and iron-based geochem-ical proxies are among the most mature available for gaininginsight into local redox conditions (18). Iron speciation measure-ments, combined with total iron to aluminum ratios (FeT/Al),have been performed on the Nonesuch Formation in the PresqueIsle Syncline and used to infer persistent water-column anoxiathroughout Paleolake Nonesuch (11). This finding was extrapo-lated to terrestrial environments as a whole in the Mesoprotero-zoic, thereby challenging the interpretation of such environmentsas a potential locus of aerobic Proterozoic eukaryotic evolution

Significance

Constraining oxygen levels and redox chemistry of Protero-zoic oceans and lakes is vital for placing environmental con-straints on early aerobic eukaryotic evolution. Recent workhas used iron-based geochemical proxies—however, interpre-tation of such measurements can be difficult due to uncertain-ties related to baselines for lake sediments and equivocal zonesassociated with empirically calibrated proxies. We integratemagnetic, geochemical, and microscale imaging techniquesto analyze the iron mineralogy of the 1.1-billion-year-oldPaleolake Nonesuch, one of the few lacustrine records of thisera. With these methods, we resolve ambiguous geochemicalsignals and document an oxycline with oxygenated shallowwaters and decreasing oxygen with depth. These results indi-cate a stable oxygenated environment in the terrestrial realm1.1 billion years ago.

Author contributions: S.P.S. and N.L.S.-H. designed research; S.P.S. and N.L.S.-H. studiedand sampled the rock cores; S.P.S. and N.L.S.-H. conducted and analyzed rock magneticexperiments; S.P.S. performed petrography and electron microscopy; E.A.S. developedgeochemical data; and S.P.S., N.L.S.-H., and E.A.S. wrote the paper.y

The authors declare no conflict of interest.y

This article is a PNAS Direct Submission.y

Published under the PNAS license.y

Data deposition: The rock magnetic data are summarized in Dataset S1. The raw rockmagnetic measurements have been deposited in the Magnetics Information Consortium(MagIC) Database (https://www.earthref.org/MagIC/doi/10.1073/PNAS.1813493115).y1 To whom correspondence should be addressed. Email: sslotz@berkeley.edu.y

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1813493115/-/DCSupplemental.y

Published online December 3, 2018.

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(11). However, published bulk-rock iron speciation data arenot entirely straightforward to interpret. The strength of theiron speciation proxy lies in its empirical calibration in modernmarine sediments, allowing for the identification of authigenicreactive iron enrichments (resulting from anoxic water columnprocesses) above an oxic baseline (the reactive iron deliveredthrough detrital processes). There are few baseline data fromlacustrine settings, and the delivery of iron to different lakes canbe highly variable (19). Further, most of the existing iron specia-tion values from the Nonesuch Formation fall not in the clearlydefined oxic or anoxic fields but in the “possibly anoxic” areaof iron speciation interpretive space (18). Given these ambigui-ties, new approaches to harness the redox information containedin the sedimentary iron record will have high utility in lacus-trine rocks and other sediments where established proxies likeiron speciation are ambiguous. This study pairs rock magnet-ics, geochemistry, and microscopy to develop a more detailedpicture of assemblages of iron oxides and sulfides in PaleolakeNonesuch. These data reveal distinct depth-dependent miner-alogical facies associated with the oxycline of this 1.1-billion-year-old lake. These facies are also seen within iron speciationextractions, but are obscured if these geochemical data are inter-preted solely on the basis of the traditional anoxia proxy ofFeHR/FeT (highly reactive iron to total iron) in conjunctionwith FeT/Al.

Paleolake NonesuchFollowing a prolonged interval of voluminous volcanic activitywithin the North American Midcontinent Rift, sedimentationwithin a thermally subsiding basin led to the deposition of sed-imentary rocks of the Oronto Group (20). The Oronto Groupcommences with the Copper Harbor Conglomerate, which rep-resents a terrestrially deposited alluvial fan and fluvial sediments(21). Locally, on the Keweenaw Peninsula, lava flows of theLake Shore Traps erupted within the Copper Harbor Con-glomerate and an andesitic lava within these flows has a U-Pbdate of 1,085.57 ± 0.25/1.3 Ma (Fig. 1) (10). The Copper Har-bor Conglomerate fines upward and is conformable with theoverlying shales, siltstones, and sandstones of the Nonesuch For-mation, which are the focus of this study. These lithologies of theNonesuch Formation are interpreted as a lacustrine facies asso-ciation (e.g., refs. 22 and 23) along a >250-km-long belt innorthern Michigan and Wisconsin (Fig. 1). Similar facies in

drill core as far south as Iowa has led to an interpretationthat Paleolake Nonesuch was >800 km long (24), althoughthe extent of these lithofacies could be due to multiple lakesalong the rift axis as in the modern East African Rift. Regard-less, the lake in northern Wisconsin and Michigan was largeand persistent with lacustrine sedimentation continuing untilafter the transition into the overlying Freda Formation. TheFreda Formation is a >4-km-thick succession that is domi-nantly composed of channelized sandstone and overbank silt-stone deposits representing a prolonged terrestrial fluvial envi-ronment (25). The Nonesuch Formation has been directlydated using Re-Os geochronology with a preferred date of1,078± 24 Ma (11). Paleomagnetic data from the NonesuchFormation (26) suggest deposition in the tropics at a latitudeof 3◦ ± 3◦.

Five drill cores from northern Wisconsin were used by ref.23 to develop a sequence stratigraphic framework for theNonesuch Formation. Our work focuses on two of these cores:DO-8 and WC-9 (Figs. 1 and 2). In this region, the transgressionthat marks the flooding surface where alluvial facies of the Cop-per Harbor Conglomerate transition to the lacustrine facies ofthe Nonesuch Formation is followed by an interval of deep waterlacustrine facies dominated by planar laminated siltstone andvery-fine sandstone with intervals of thinly interbedded siltstoneand carbonate (Fig. 2) (23). Following the maximum floodingof the lake, an aggradational–progradational sequence recordsa progressive shallowing sequence (Fig. 2). The Nonesuch For-mation is transitional with the overlying Freda Formation andthe formation boundary is typically set on the basis of color(23, 25). As a result, similar lithofacies deposited in a lacustrineenvironment are found on either side of the formation bound-ary, with fluvial channel sandstone present higher in the Fredastratigraphy (Fig. 2).

That the Nonesuch Formation is conformable with underly-ing and overlying terrestrial sediments has been interpreted toimply that it was deposited in a terrestrial lake rather than amarine setting (e.g., refs. 22 and 27). However, the Nonesuchfacies themselves could be consistent with either a lacustrineor a protected marine depositional environment. Some workershave invoked incursion of marine waters into the basin basedon interpretations of the affinity of putative sterane biomarkers(data from ref. 28; the indigenous origin is called into questionby data from ref. 29 revealing modern contamination) and the

Fig. 1. Geologic map and summary stratigraphy of the Nonesuch Formation and other units from within the Midcontinent Rift. The geological data on themap are from ref. 9 and the stratigraphic column from ref. 10. The Re-Os date for the Nonesuch Formation is shown in italics with 2σ uncertainty (11). TheU-Pb dates (10, 12) are shown with 2σ uncertainties (X/Y) that include analytical uncertainty alone (X) and include tracer and decay constant uncertainty (Y)for comparison with the Re-Os date. Cores studied in this work from the Ashland Syncline (DO-8 and WC-9) are shown with red stars.

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Fig. 2. (A and B) Rock magnetic and iron speciation data from the DO-8 (A) and WC-9 (B) cores through the Nonesuch Formation. Lithostratigraphy ismodified from ref. 23 with additional measured section. Core depth is given in feet (1 foot = 0.3048 m) as those units are used for depth in the originalcores. The grain size abbreviations are as follows: C, coarse sandstone; F, fine sandstone; G, granule to cobble conglomerate; M, medium sandstone; Mu,mud; S, silt; Vf, very fine sandstone. The sequence stratigraphic interpretation of deepening and shallowing follows refs. 22 and 23. Data points arecolored, reflecting the actual rock color. On the Bcr (coercivity of remanence) plot, the average coercivity values of hematite and magnetite are plottedfor reference. Magnetite abundance is an upper bound calculated using the saturation magnetization value of well-characterized magnetite grains (39).Hematite abundance was calculated using either the saturation magnetization (when it is the only ferromagnetic phase present) or the remanent saturationmagnetization and coercivity spectra. Range bars on these calculated values capture both 1 standard deviation uncertainty associated with coercivity spectraunmixing and the range of (remanent) saturation magnetization values measured in pure minerals (40, 41) (for more details see SI Appendix). The threedistinct magnetic facies, composed of distinct magnetic mineral assemblages, are denoted by blue lines and labeled along with interpreted depositionalredox conditions. In the FeHR/FeT and Fepy/FeHR plots (HR, highly reactive; py, pyrite; T, total), vertical dashed lines denote boundaries for oxic vs. anoxicand ferruginous vs. euxinic water column conditions, respectively, that are used for iron speciation paleoredox proxy interpretations (18). While many ofthe FeHR/FeT data fall in the “equivocal” zone between anoxic and oxic conditions, the iron removed through each progressive extraction, particularly theFedithionite extraction, varies with the magnetic facies.

presence of sulfides indicative of bacterial sulfate reduction (30).Given that neither of these lines of evidence is diagnostic of amarine environment, the stratigraphic context of the formationand its position within an intracontinental rift favor a lacustrinedepositional setting (22).

The Nonesuch Formation is exceptionally well preserved,with maximum burial temperatures of 140–150 ◦C estimatedby modeled burial temperatures (31) and 125–155 ◦C inferredfrom solid-state reordering modeling of clumped isotope val-ues (32). In contrast to the Iron River Syncline and PresqueIsle Syncline (>100 km away; Fig. 1) where there is coppermineralization in the basal Nonesuch Formation, there is nomineralization in cores from the Ashland Syncline (23) (SIAppendix, Fig. S1).

Iron Speciation ResultsGeochemical analyses were performed to compare the DO-8 andWC-9 cores to previously published analyses from the None-such Formation within the Presque Isle Syncline (Fig. 1). Ironspeciation is a bulk sequential extraction technique that sepa-rates iron into distinct pools that are ratioed and compared withempirical calibrations on modern sediments to make interpre-tations of paleoredox conditions (18, 33). Analyses proceededusing standard protocols (33, 34) with measurements of stan-dards consistent with those of previous analyses (35). Our resultsfor FeHR/FeT are generally below the common threshold of 0.38used to separate likely oxic (<0.38) from anoxic (>0.38) depo-sitional environments (18, 36) such that they fall in the rangeof equivocal values (0.2–0.38) or the oxic range (<0.2; Fig. 2).

12940 | www.pnas.org/cgi/doi/10.1073/pnas.1813493115 Slotznick et al.

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Samples falling in the equivocal zone could have been depositedunder an oxygenated water column or could have been depositedin anoxic conditions but with processes masking FeHR enrich-ment. Such processes could include rapid sedimentation or burialdiagenesis/metamorphism that transformed highly reactive ironminerals into unreactive phases such as clay minerals (37, 38).The Fepy/FeHR (pyrite iron to highly reactive iron) is elevatedin lower portions of the formation, but still indicates that notall reactive iron was pyritized, similar to the findings fromthe Presque Isle Syncline (11). Overall, these iron speciationratios are ambiguous and elude straightforward interpretation ofpaleoredox.

Magnetic and Petrographic Results with InterpretationExperimentally determined estimates of magnetization and coer-civity on samples spanning the stratigraphic sections (Figs. 2and 3) reveal three distinct magnetic facies within the NonesuchFormation. Low-temperature magnetic experiments designed toelucidate low-temperature transitions confirm the ferromagneticmineral identifications associated with these facies in both cores(SI Appendix, Figs. S2 and S3). Petrographic and microscaletextural geochemical analyses on selected samples using trans-mitted light, reflected light, and electron microscopy pairedwith energy-dispersive X-ray spectroscopy further confirm themagnetic mineralogy interpretations and give a more completeperspective of the mineralogy associated with each facies and thedepositional and diagenetic processes they represent (Fig. 3 andSI Appendix, Figs. S7–S9).

Magnetic facies 1 is present in the deepest water lithologicfacies and is characterized by a lack of hematite and a very

weak magnetization carried by trace magnetite (Figs. 2 and 3and SI Appendix, Figs. S1–S3). No magnetite could be seen usingmicroscopy techniques, corroborating the low abundance and/ornanoscale size of these minerals. The iron within this facies ispredominantly found in phyllosilicates, calcalumnosilicates, andabundant sulfides (Fig. 3 and SI Appendix, Fig. S7). Euhedralpyrite crystals range in size from <1 µm to 15 µm and can formaggregates up to 100 µm. Based on their shape and occasionaltextural association with iron-bearing clays, we interpret them tohave formed in pore fluids from iron liberated from magnetiteand clays during reductive dissolution and sulfidization (Fig. 3and SI Appendix, Fig. S7). Facies 1 shows no evidence for oxida-tion of these reduced phases, highlighting the excellent preserva-tion of these drill cores and lack of secondary oxidative fluid flowin this region. Titanium minerals—titanium oxide (rutile and/oranatase), leucoxene, and titanite—are found in samples of thisfacies, and texturally some appear to be authigenic (SI Appendix,Fig. S7). Authigenic titanium-bearing minerals commonly formduring dissolution of iron-bearing phases including titanomag-netite grains (42–44). Taken together, these data indicate thatthe very weak magnetization relative to the other magnetic faciesis the result of reductive dissolution of iron oxides, likely througha combination of dissimilatory iron reduction and sulfidization.

Magnetic facies 2 is characterized by a mixed assemblage ofmagnetite and hematite with relatively strong overall magneti-zation (Figs. 2 and 3 and SI Appendix, Figs. S1–S3). Microscaletextural observations demonstrate the presence of detrital(titano)magnetite with igneous origins based on the exsolutionbetween titanomagnetite and ilmenite (Fig. 3 and SI Appendix,Fig. S8). A sharply preserved Verwey transition revealed through

Fig. 3. Example backscatter electron microscope images and bulk coercivity spectra from each magnetic facies. The coercivity spectra show the derivativeof magnetization (dM (Am2)/dB (mT)) as a function of applied field and are fitted with log-Gaussian components (40). The multiple components in facies2 are unmixed and shown with uncertainty associated with the unmixing (40). Facies 1 is characterized by the presence of pyrite and a noisy coercivityspectrum due to weak magnetization that indicates the presence of magnetite in trace quantities (∼15 ppm in this sample). Facies 2 has detrital grains oftitanomagnetite (igneous titanomagnetite with exsolution lamallae visible in image) and hematite; significant quantities of both magnetite and hematiteresult in a double-peaked coercivity spectrum. Facies 3 has a coercivity spectrum dominated by hematite with disseminated hematite, aggregates of hematitecrystals, and oxidized detrital titanomagnetite grains visible via electron microscopy. Mineral abbreviations are as follows: K-spar, potassium feldspar; andTi-Maghem, titanomaghemite.

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low-temperature magnetometry indicates the presence of mag-netite with no to minimal oxidation (SI Appendix, Figs. S2 andS3). The relatively high coercivity of the magnetite as revealedin coercivity spectra (Fig. 3), combined with the results of first-order reversal curve experiments (SI Appendix, Fig. S5), is con-sistent with behavior dominated by small (<3 µm) vortex-stategrains (45). Detrital grains containing hematite are observed;the hematite is typically associated with, and replacing, phyl-losilicates, indicating oxidation during pretransport weathering,riverine transport, and/or deposition (Fig. 3 and SI Appendix,Fig. S8). The detrital nature of these grains is confirmed by theirsometimes rounded shapes and the deformation of clay mineralsbetween them and other detrital grains (SI Appendix, Fig. S8).Pieces of organic matter (80–100 µm by 10 µm) with compactionwarping are also preserved within this facies (SI Appendix, Fig.S8). Some reductive dissolution of iron oxides may have occurredbased on the presence of mixed mineral grains of titanite andiron oxides as well as minor amounts of pyrite (SI Appendix, Fig.S8). However, in contrast with facies 1, the data show that suchreductive dissolution was minimal and likely isolated to smallregions of pore waters that became anoxic and sulfidic withinthe sediment. We interpret this mixed hematite and magnetiteassemblage as a good representation of the detrital riverine inputto the lake given that iron oxide grains were largely not reduc-tively dissolved nor were magnetite grains fully oxidized to ferricoxide phases. The preservation of a detrital assemblage is there-fore more consistent with persistent intermediate oxygen levelsthan with fluctuations between anoxic and oxic conditions.

Magnetic facies 3 is present in the shallowest water sedi-ments and is dominated by hematite with a minimal contri-bution from lower coercivity phases such as magnetite (Figs.2 and 3 and SI Appendix, Figs. S2 and S3). Microscale tex-tural analyses reveal hematite that formed from oxidationof detrital igneous (titano)magnetite grains based on abun-dant titanohematite/titanomaghemite, sometimes within rutilegrains or with relict skeletal and trellis lamallae shapes (Fig. 3and SI Appendix, Fig. S9). Additional hematite and titanohe-matite/titanomaghemite grains are seen as platelets within phyl-losilicate grains or rimming quartz grains (SI Appendix, Fig.S9). Titanite and leucoxene are frequently observed and authi-genic titanium oxide grains were found, confirming that in-placeoxidation of iron–titanium minerals occurred (Fig. 3 and SIAppendix, Fig. S9) (42). Petrography also shows that, in additionto the observed grains of hematite, there is abundant pigmentaryhematite in facies 3 (SI Appendix, Fig. S9). These data reveal thatin this facies there has been significant oxidation of the detritalinput to the lake both during transport and, due to the presenceof pigmentary hematite and authigenic titanium oxides, withinthe sediment. The original detrital input appears to have beensimilar to facies 2 before additional oxidation.

Combined Insights from Magnetism and Microscale TexturalAnalyses with Iron SpeciationWhile rock magnetism and petrography reveal that the ironmineralogy (and interpreted paleoredox) changes significantlythrough the cores, the FeHR/FeT ratios are rather uniform. Theprevious iron speciation study of Lake Nonesuch (11) inter-preted their similar equivocal FeHR/FeT ratios to be indicativeof ferruginous environmental conditions obscured by postdepo-sitional transformation of the FeHR pool into clays. A majordriver of this interpretation was elevated FeT/Al ratios abovenormal oxic shale values (e.g., 0.53 ± 0.11) (46) and elevatediron abundance in poorly reactive silicates. Both of these enrich-ments were proposed to result from iron shuttling under anoxicconditions (11). The FeT/Al ratios determined for our sam-ples (ranging from 0.57 to 0.99) are similar to these previousresults (SI Appendix, Fig. S1). However, maps of modern soil geo-chemistry reveal there is substantial variability in detrital FeT/Al

ratios (19) and a lake setting within a volcanic province, suchas Paleolake Nonesuch, is a setting where the FeT/Al ratio ofthe detrital flux is likely to be higher than average. The FeT/Alratios in the oxic shallow sediments of facies 3, which should nothave experienced authigenic enrichments, are similar to thosewithin the other facies (SI Appendix, Fig. S1). These data sup-port an interpretation that the FeT/Al ratio is representative ofthe detrital flux into Paleolake Nonesuch rather than the resultof enrichment of iron from an anoxic water column.

Although the FeHR/FeT ratios themselves may be ambigu-ous and lack a clear correlation to the three magnetic facies,if the iron extraction pools are looked at in detail, consider-able differences are noted, which also separate the sections intothree facies (Fig. 2 and SI Appendix, Fig. S1). Facies 3 standsout as having high Fedithionite, low Feacetate, and low FeCRS

[chromium reducible sulfur (CRS)]; the dithionite extractiontargets ferric iron (hydr)oxides, which agrees well with our mag-netic quantification of abundant hematite in this facies. Whilehematite is the most oxidized endmember within the ulvospinel–magnetite–ilmenite–hematite series and forms within oxidizingenvironments, in iron speciation analysis it is grouped withthe highly reactive phases. Therefore, its presence increasesFeHR/FeT, which is higher in oxic facies 3 than in the other faciesand contains the only sample with FeHR/FeT > 0.38. This highFeHR/FeT value could erroneously lead to the interpretation ofan anoxic environment if the mineralogy leading to this highvalue were not considered. Facies 1 has relatively low Fedithionite

and high FeCRS, which agrees with magnetic analyses suggestingno hematite and petrographic observations of pyrite (extractedas CRS) in these samples. While the Feoxalate pool is typicallyattributed to magnetite such that it is sometimes called Femag

(33), facies 1, 2, and 3 have similar Feoxalate values even thoughthe magnetic and textural analyses show facies 2 to have muchmore magnetite. The quantity of iron in the oxalate extraction isone to three orders of magnitude higher than the abundance ofmagnetite calculated using the magnetic data (SI Appendix, Fig.S10), and quantities of magnetite approaching 1 wt% in a shalewould be highly unusual. These results indicate that the oxalateextraction is solubilizing iron from other minerals in additionto magnetite. More research is needed to fully understand themineralogy removed by each sequential extraction step in nat-ural samples. In the meantime, workers should continue to usecareful terminology to make it clear that this is an operationaldefinition and that much of the iron in the oxalate extractionis not from magnetite. Considering the sequential iron specia-tion extraction data from ref. 11 in a similar framework wouldplace most of those analyses in facies 1 with zones of facies 3and 2 near the basal flooding surface with the Copper HarborConglomerate. As less than half of the thickness of the None-such Formation was captured in the Presque Isle drill core∗

from which these prior data were obtained, such a classificationmatches our interpretations.

A Preserved OxyclineThese three facies and their juxtaposition can be explained as theresult of an oxycline in Paleolake Nonesuch. The detrital inputto the lake included both magnetite and hematite (preserved infacies 2) due to weathering and oxidation of the source igneousmaterial during transport. Sediments in the deepest part of thelake were anoxic, possibly with anoxia extending into the watercolumn. As a result, delivered iron oxides underwent reductivedissolution through microbial metabolic processes as recorded byfacies 1. Much of this iron and iron within sheet silicates reactedwith sulfide to form pyrite, but sulfide availability was restricted

*The 1958 Bear Creek Mining Company PI-1 and PI-2 drill logs are archived and availableby request from the Northern Michigan Geologic Core and Sample Repository.

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to pore waters and not sufficient to sulfidize all of the availablereactive iron. Intermediate oxygen levels in waters throughoutmuch of the lake allowed for the preservation of detrital mag-netite and hematite in facies 2. In the shallow waters of the lakerecorded in facies 3, oxic conditions prevailed and most of thedetrital magnetite, as well as iron in other phases, was oxidizedto hematite.

We interpret this vertical sequence of facies to reflect astacking of laterally distributed environments such that thetransition from the deepest-water low–iron-oxide facies intothe intermediate-water magnetite-rich facies and the shallower-water hematite-rich facies is the result of an oxycline within theancient lake. The depth dependence of the oxycline is similarto that found in modern eutrophic lakes wherein the aerobicrespiration of descending organic matter leads to a decrease indissolved oxygen with depth. Overall, these data indicate thatthe lake was more deeply oxygenated than has previously beeninterpreted on the basis of iron speciation data alone. For muchof its temporal duration, and throughout much of its water col-umn, there was oxygen in the waters of Paleolake Nonesuch.While trophic modes are poorly known for the diverse biota

found within Nonesuch (4) and their paleobathymetric distribu-tion is not well constrained, these eukaryotes lived in a stable andhospitable lake environment with available oxygen. It remainsa puzzle why these eukaryotic denizens preserved in the fossilrecord did not leave an appreciable sterane record (29) and why,despite seemingly favorable environmental conditions, eukary-otic productivity was so low that sterane/hopane ratios have beenfound to be zero in indigenous organic matter (47). Regard-less, the environmental signal from this diverse lacustrine fossillocality is becoming clear. Overall, these results highlight thatcoupling magnetic and microscale textural data with geochemicaldata can resolve ambiguous redox interpretations in deep time.

ACKNOWLEDGMENTS. Esther Stewart and Valerie Stanley provided supportand guidance at the Wisconsin Geological and Natural History Survey corerepository. Luke Fairchild assisted with geological data compilation; SabrinaTecklenburg assisted with iron speciation analyses; and Malcolm Hodgskiss,Ioan Lascu, and Bruce Moskowitz provided useful insights regarding inter-pretations. This research was supported by the Esper S. Larsen Jr. ResearchFund, a Sloan Research Fellowship, and the Miller Institute for Basic Science.Many of the rock magnetic experiments were conducted during a visitingfellowship at the Institute for Rock Magnetism which is supported by theNational Science Foundation and the University of Minnesota.

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