Oxygenated Mesoproterozoic Lake Revealed Through Magnetic

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Erik and , | 1073/pnas.1813493115/-/DCSupplemental hsatcecnan uprigifrainoln at online information supporting contains article This 1 Consortium Information Magnetics (https://www.earthref.org/MagIC/doi/10.1073/PNAS.1813493115).y in the Database in (MagIC) summarized deposited are been data have measurements magnetic magnetic rock The deposition: Data the under Published Submission.y Direct PNAS a is article This interest.y of conflict no declare paper.y authors the developed The wrote E.A.S. E.A.S. and magnetic microscopy; N.L.S.-H., rock S.P.S., electron and analyzed data; and and geochemical conducted petrography N.L.S.-H. performed studied and S.P.S. S.P.S. N.L.S.-H. cores; experiments; and rock S.P.S. the research; sampled designed and N.L.S.-H. and S.P.S. contributions: Author evolution eukaryotic Proterozoic aerobic of locus potential a environments Mesoprotero- such as of the interpretation in the whole challenging thereby a zoic, as extrapo- environments anoxia was terrestrial finding water-column to This lated persistent (11). infer Nonesuch to Paleolake throughout used (Fe and Presque the ratios Syncline in aluminum Formation Isle Nonesuch to the on gaining iron performed been for total have available with measure- speciation combined mature Iron (18). ments, most conditions redox the local geochem- into among insight iron-based are and proxies interwoven, ical tightly are environment equivocal cycling oxidative is without arise interpretation oxidative 17). can (16, from this fractionation such frac- resulted However, that Torridon have given isotope (15). and to sulfur cycling Stoer interpreted large the sulfur were of on that rocks Mesoproterozoic based groups sedimentary the proposed from oxygena- during been tionations Early environments also (4). lacustrine has ones hospitable marine of more is than been tion evolution record have environments eukaryotic may This lacustrine waters to that oxygenated (4). interpretation stable species the with to different assemblages, leads pres- marine 50 similar-aged which the than diverse than more indicate be to to more argued interpreted of further ence been has Formation owo orsodnesol eadesd mi:[email protected] Email: addressed. be should correspondence whom To aeasal xgntdevrneti h ersra realm ago. terrestrial years the billion in 1.1 environment indi- oxygenated results stable These a shallow depth. cate with oxygenated oxygen with decreasing oxycline and waters an geochemical document ambiguous 1.1-billion-year-old and resolve this signals we of the methods, records these lacustrine of With few era. the mineralogy of one iron Nonesuch, techniques Paleolake the imaging analyze microscale integrate to and We geochemical, proxies. calibrated magnetic, empirically with zones equivocal associated and sediments uncertain- lake for to baselines due to difficult related ties be can measurements work such of Recent interpre- tation proxies—however, evolution. geochemical con- iron-based eukaryotic environmental used aerobic placing has for early vital Protero- on is of straints lakes chemistry and redox oceans and zoic levels oxygen Constraining Significance h hmsr n ieaoyo rnadoye nthe in oxygen and iron of mineralogy and chemistry The b b eateto elgclSine,Safr University, Stanford Sciences, Geological of Department NSlicense.y PNAS . y www.pnas.org/lookup/suppl/doi:10. 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EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES (11). However, published bulk-rock iron speciation data are drill core as far south as Iowa has led to an interpretation not entirely straightforward to interpret. The strength of the that Paleolake Nonesuch was >800 km long (24), although iron speciation proxy lies in its empirical calibration in modern the extent of these lithofacies could be due to multiple lakes marine sediments, allowing for the identification of authigenic along the rift axis as in the modern East African Rift. Regard- reactive iron enrichments (resulting from anoxic water column less, the lake in northern Wisconsin and Michigan was large processes) above an oxic baseline (the reactive iron delivered and persistent with lacustrine sedimentation continuing until through detrital processes). There are few baseline data from after the transition into the overlying Freda Formation. The lacustrine settings, and the delivery of iron to different lakes can Freda Formation is a >4-km-thick succession that is domi- be highly variable (19). Further, most of the existing iron specia- nantly composed of channelized sandstone and overbank silt- tion values from the Nonesuch Formation fall not in the clearly stone deposits representing a prolonged terrestrial fluvial envi- defined oxic or anoxic fields but in the “possibly anoxic” area ronment (25). The Nonesuch Formation has been directly of iron speciation interpretive space (18). Given these ambigui- dated using Re-Os geochronology with a preferred date of ties, new approaches to harness the redox information contained 1,078 ± 24 Ma (11). Paleomagnetic data from the Nonesuch in the sedimentary iron record will have high utility in lacus- Formation (26) suggest deposition in the tropics at a latitude trine rocks and other sediments where established proxies like of 3◦ ± 3◦. iron speciation are ambiguous. This study pairs rock magnet- Five drill cores from northern Wisconsin were used by ref. ics, geochemistry, and microscopy to develop a more detailed 23 to develop a sequence stratigraphic framework for the picture of assemblages of iron oxides and sulfides in Paleolake Nonesuch Formation. Our work focuses on two of these cores: Nonesuch. These data reveal distinct depth-dependent miner- DO-8 and WC-9 (Figs. 1 and 2). In this region, the transgression alogical facies associated with the oxycline of this 1.1-billion- that marks the flooding surface where alluvial facies of the Cop- year-old lake. These facies are also seen within iron speciation per Harbor Conglomerate transition to the lacustrine facies of extractions, but are obscured if these geochemical data are inter- the Nonesuch Formation is followed by an interval of deep water preted solely on the basis of the traditional anoxia proxy of lacustrine facies dominated by planar laminated siltstone and FeHR/FeT (highly reactive iron to total iron) in conjunction very-fine sandstone with intervals of thinly interbedded siltstone with FeT/Al. and carbonate (Fig. 2) (23). Following the maximum flooding of the lake, an aggradational–progradational sequence records Paleolake Nonesuch a progressive shallowing sequence (Fig. 2). The Nonesuch For- Following a prolonged interval of voluminous volcanic activity mation is transitional with the overlying Freda Formation and within the North American Midcontinent Rift, sedimentation the formation boundary is typically set on the basis of color within a thermally subsiding basin led to the deposition of sed- (23, 25). As a result, similar lithofacies deposited in a lacustrine imentary rocks of the Oronto Group (20). The Oronto Group environment are found on either side of the formation bound- commences with the Copper Harbor Conglomerate, which rep- ary, with fluvial channel sandstone present higher in the Freda resents a terrestrially deposited alluvial fan and fluvial sediments stratigraphy (Fig. 2). (21). Locally, on the Keweenaw Peninsula, lava flows of the That the Nonesuch Formation is conformable with underly- Lake Shore Traps erupted within the Copper Harbor Con- ing and overlying terrestrial sediments has been interpreted to glomerate and an andesitic lava within these flows has a U-Pb imply that it was deposited in a terrestrial lake rather than a date of 1,085.57 ± 0.25/1.3 Ma (Fig. 1) (10). The Copper Har- marine setting (e.g., refs. 22 and 27). However, the Nonesuch bor Conglomerate fines upward and is conformable with the facies themselves could be consistent with either a lacustrine overlying shales, siltstones, and sandstones of the Nonesuch For- or a protected marine depositional environment. Some workers mation, which are the focus of this study. These lithologies of the have invoked incursion of marine waters into the basin based Nonesuch Formation are interpreted as a lacustrine facies asso- on interpretations of the affinity of putative sterane biomarkers ciation (e.g., refs. 22 and 23) along a >250-km-long belt in (data from ref. 28; the indigenous origin is called into question northern Michigan and Wisconsin (Fig. 1). Similar facies in by 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 the map 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). The U-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.

2 of 6 | www.pnas.org/cgi/doi/10.1073/pnas.1813493115 Slotznick et al. Downloaded by guest on September 24, 2021 Downloaded by guest on September 24, 2021 u;S it f eyfiesnsoe h eunesrtgahcitrrtto fdeeigadsalwn olw es 2ad2.Dt onsare points Data 23. and 22 original refs. the follows in shallowing Mu, depth sandstone; and for medium used deepening M, are of conglomerate; units interpretation cobble those B to stratigraphic as the granule m) sequence On G, 0.3048 color. The sandstone; = rock fine sandstone. foot F, actual (1 fine sandstone; the feet coarse very reflecting in C, Vf, colored, given follows: silt; is as depth S, are Core abbreviations mud; section. size measured grain additional The with cores. 23 ref. from modified 2. Fig. n ergnu s uii ae ouncniin,rsetvl,ta r sdfrio pcainploeo rx nepeain 1) hl ayof many While (18). interpretations proxy paleoredox speciation depositional iron interpreted for with used along are labeled that Fe respectively, and see conditions, lines details column blue Fe more water by the (for euxinic denoted 41) vs. are (40, ferruginous assemblages, minerals Fe and mineral pure the magnetic in spectra In coercivity distinct measured conditions. with values of associated redox magnetization (39). composed uncertainty saturation deviation saturation grains remanent facies, standard the (remanent) magnetite magnetic 1 or both of well-characterized distinct present) capture phase range of values ferromagnetic the calculated value only these and the magnetization on is unmixing saturation bars it Range (when the spectra. magnetization using coercivity saturation and calculated the magnetization either bound using upper calculated was an abundance is Hematite abundance Magnetite reference. for ieaiaini oe rmteAhadSnln 2)(SI al. et (23) Slotznick Syncline Ashland copper no the is is there there from S1). Fig. Formation, where Presque cores Appendix, Nonesuch 1) and in basal Syncline Fig. mineralization the River away; in Iron km the mineralization (>100 val- to Syncline isotope contrast clumped Isle In of (32). modeling ues 125–155 reordering and solid-state (31) 140–150 from temperatures of burial modeled temperatures by burial maximum with lacustrine a favor rift a (22). intracontinental setting of an formation depositional diagnostic within the is position of its evidence context and of stratigraphic lines the environment, these marine of neither (30). reduction that sulfate bacterial Given of indicative sulfides of presence B A dithionite h oeuhFraini xetoal elpreserved, well exceptionally is Formation Nonesuch The HR (A /Fe xrcin aiswt h antcfacies. magnetic the with varies extraction, T and aafl nte“qioa”zn ewe nxcadoi odtos h rnrmvdtruhec rgesv xrcin atclrythe particularly extraction, progressive each through removed iron the conditions, oxic and anoxic between zone “equivocal” the in fall data oe hog h oeuhFrain ihsrtgah is Lithostratigraphy Formation. Nonesuch the through cores (B) WC-9 and (A) DO-8 the from data speciation iron and magnetic Rock B) HR /Fe T n Fe and py /Fe HR lt H,hgl ecie y yie ,ttl,vria ahdlnsdnt onaisfroi s anoxic vs. oxic for boundaries denote lines dashed vertical total), T, pyrite; py, reactive; highly (HR, plots cr cecvt frmnne lt h vrg oriiyvle fhmtt n antt r plotted are magnetite and hematite of values coercivity average the plot, remanence) of (coercivity ◦ estimated C ◦ inferred C .;Fg 2). Fig. (<0.2; range range the oxic in the fall or they (0.2–0.38) that depo- values such (>0.38) equivocal anoxic 36) of from (18, (<0.38) environments oxic sitional likely separate to stan- used results of Our (35). measurements analyses Fe previous for with of those proceeded 34) with Analyses consistent (33, dards 33). protocols (18, interpre- standard make conditions using to paleoredox sediments with of compared modern tations sepa- and on ratioed that are calibrations technique that Iron empirical pools extraction distinct 1). into sequential (Fig. None- iron bulk Syncline rates the a Isle from is Presque analyses the speciation within published Formation previously such to and cores DO-8 the WC-9 compare to performed were analyses Geochemical Results Speciation Iron HR /Fe T r eeal eo h omntrsodo 0.38 of threshold common the below generally are NSLts Articles Latest PNAS IAppendix SI .Tethree The ). | f6 of 3

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES Samples falling in the equivocal zone could have been deposited weak magnetization carried by trace magnetite (Figs. 2 and 3 under an oxygenated water column or could have been deposited and SI Appendix, Figs. S1–S3). No magnetite could be seen using in anoxic conditions but with processes masking FeHR enrich- microscopy techniques, corroborating the low abundance and/or ment. Such processes could include rapid sedimentation or burial nanoscale size of these minerals. The iron within this facies is diagenesis/metamorphism that transformed highly reactive iron predominantly found in phyllosilicates, calcalumnosilicates, and minerals into unreactive phases such as clay minerals (37, 38). abundant sulfides (Fig. 3 and SI Appendix, Fig. S7). Euhedral The Fepy/FeHR (pyrite iron to highly reactive iron) is elevated pyrite crystals range in size from <1 µm to 15 µm and can form in lower portions of the formation, but still indicates that not aggregates up to 100 µm. Based on their shape and occasional all reactive iron was pyritized, similar to the findings from textural association with iron-bearing clays, we interpret them to the Presque Isle Syncline (11). Overall, these iron speciation have formed in pore fluids from iron liberated from magnetite ratios are ambiguous and elude straightforward interpretation of and clays during reductive dissolution and sulfidization (Fig. 3 paleoredox. and SI Appendix, Fig. S7). Facies 1 shows no evidence for oxida- tion of these reduced phases, highlighting the excellent preserva- Magnetic and Petrographic Results with Interpretation tion of these drill cores and lack of secondary oxidative fluid flow Experimentally determined estimates of magnetization and coer- in this region. Titanium minerals—titanium oxide (rutile and/or civity on samples spanning the stratigraphic sections (Figs. 2 anatase), leucoxene, and titanite—are found in samples of this and 3) reveal three distinct magnetic facies within the Nonesuch facies, and texturally some appear to be authigenic (SI Appendix, Formation. Low-temperature magnetic experiments designed to Fig. S7). Authigenic titanium-bearing minerals commonly form elucidate low-temperature transitions confirm the ferromagnetic during dissolution of iron-bearing phases including titanomag- mineral identifications associated with these facies in both cores netite grains (42–44). Taken together, these data indicate that (SI Appendix, Figs. S2 and S3). Petrographic and microscale the very weak magnetization relative to the other magnetic facies textural geochemical analyses on selected samples using trans- is the result of reductive dissolution of iron oxides, likely through mitted light, reflected light, and electron microscopy paired a combination of dissimilatory iron reduction and sulfidization. with energy-dispersive X-ray spectroscopy further confirm the Magnetic facies 2 is characterized by a mixed assemblage of magnetic mineralogy interpretations and give a more complete magnetite and hematite with relatively strong overall magneti- perspective of the mineralogy associated with each facies and the zation (Figs. 2 and 3 and SI Appendix, Figs. S1–S3). Microscale depositional and diagenetic processes they represent (Fig. 3 and textural observations demonstrate the presence of detrital SI Appendix, Figs. S7–S9). (titano)magnetite with igneous origins based on the exsolution Magnetic facies 1 is present in the deepest water lithologic between titanomagnetite and ilmenite (Fig. 3 and SI Appendix, facies and is characterized by a lack of hematite and a very 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 derivative of magnetization (dM (Am2)/dB (mT)) as a function of applied field and are fitted with log-Gaussian components (40). The multiple components in facies 2 are unmixed and shown with uncertainty associated with the unmixing (40). Facies 1 is characterized by the presence of pyrite and a noisy coercivity spectrum due to weak magnetization that indicates the presence of magnetite in trace quantities (∼15 ppm in this sample). Facies 2 has detrital grains of titanomagnetite (igneous titanomagnetite with exsolution lamallae visible in image) and hematite; significant quantities of both magnetite and hematite result in a double-peaked coercivity spectrum. Facies 3 has a coercivity spectrum dominated by hematite with disseminated hematite, aggregates of hematite crystals, and oxidized detrital titanomagnetite grains visible via electron microscopy. Mineral abbreviations are as follows: K-spar, potassium feldspar; and Ti-Maghem, titanomaghemite.

4 of 6 | www.pnas.org/cgi/doi/10.1073/pnas.1813493115 Slotznick et al. 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EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES to pore waters and not sufficient to sulfidize all of the available found within Nonesuch (4) and their paleobathymetric distribu- reactive iron. Intermediate oxygen levels in waters throughout tion is not well constrained, these eukaryotes lived in a stable and much of the lake allowed for the preservation of detrital mag- hospitable lake environment with available oxygen. It remains netite and hematite in facies 2. In the shallow waters of the lake a puzzle why these eukaryotic denizens preserved in the fossil recorded in facies 3, oxic conditions prevailed and most of the record did not leave an appreciable sterane record (29) and why, detrital magnetite, as well as iron in other phases, was oxidized despite seemingly favorable environmental conditions, eukary- to hematite. otic productivity was so low that sterane/hopane ratios have been We interpret this vertical sequence of facies to reflect a found to be zero in indigenous organic matter (47). Regard- stacking of laterally distributed environments such that the less, the environmental signal from this diverse lacustrine fossil transition from the deepest-water low–iron-oxide facies into locality is becoming clear. Overall, these results highlight that the intermediate-water magnetite-rich facies and the shallower- coupling magnetic and microscale textural data with geochemical water hematite-rich facies is the result of an oxycline within the data can resolve ambiguous redox interpretations in deep time. ancient lake. The depth dependence of the oxycline is similar to that found in modern eutrophic lakes wherein the aerobic ACKNOWLEDGMENTS. Esther Stewart and Valerie Stanley provided support and guidance at the Wisconsin Geological and Natural History Survey core respiration of descending organic matter leads to a decrease in repository. Luke Fairchild assisted with geological data compilation; Sabrina dissolved oxygen with depth. Overall, these data indicate that Tecklenburg assisted with iron speciation analyses; and Malcolm Hodgskiss, the lake was more deeply oxygenated than has previously been Ioan Lascu, and Bruce Moskowitz provided useful insights regarding inter- interpreted on the basis of iron speciation data alone. For much pretations. This research was supported by the Esper S. Larsen Jr. Research Fund, a Sloan Research Fellowship, and the Miller Institute for Basic Science. of its temporal duration, and throughout much of its water col- Many of the rock magnetic experiments were conducted during a visiting umn, there was oxygen in the waters of Paleolake Nonesuch. fellowship at the Institute for Rock Magnetism which is supported by the While trophic modes are poorly known for the diverse biota National Science Foundation and the University of Minnesota.

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