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Seawater-buffered diagenesis, destruction of carbon isotope excursions, and the composition of DIC in oceans

Paul F. Hoffmana,b,1 and Kelsey G. Lamothec

aSchool of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8P 5C2, Canada; bDepartment of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138; and cDepartment of Earth and Planetary Sciences, McGill University, Montreal, QC H3A 0E8, Canada

Edited by Mark H. Thiemens, University of California San Diego, La Jolla, CA, and approved August 16, 2019 (received for review June 4, 2019) Carbonate sediments of nonglacial (659 to 649 Ma) permeable lithologies on the foreslopes of carbonate platforms and early (635 to 590 Ma) age exhibit large positive and (Fig. 2 B and C), where seawater invasion is a response to geo- δ13 negative Ccarb excursions in a shallow-water marine platform in thermally driven porewater convection (17–21). In platform in- 13 northern . The same excursions are recorded in fringing teriors, where most Neoproterozoic δ Ccarb records have been deep-sea fans and in carbonate platforms on other paleoconti- obtained, diagenesis becomes increasingly sediment buffered with nents. However, coeval carbonates in the upper foreslope of the 13 distance along pore-fluid pathways (12). In these areas, δ Ccarb Namibian platform, and to a lesser extent in the outermost plat- values of the sediment tend to be preserved. δ13 form, have relatively uniform Ccarb compositions compatible Although C is more abundant in carbonate sediment, relative with dissolved inorganic carbon (DIC) in the modern ocean. We to seawater, than is Ca or Mg, seawater-buffered diagenesis may attribute the uniform values to fluid-buffered diagenesis that oc- 13 be capable of altering δ Ccarb toward the range of open-ocean curred where seawater invaded the sediment in response to geo- 13 13 δ CDIC. If complete reequilibration occurred, the altered δ Ccarb thermal porewater convection. This attribution, which is testable δ13 δ13 would be a record of open-ocean CDIC. Our strategy, therefore, with paired Ca and Mg isotopes, implies that large Ccarb excur- was to compare δ13C records from the interior of the Otavi/ sions observed in Neoproterozoic platforms, while sedimentary in carb Swakop platform, where diagenesis is expected to have been origin, do not reflect the composition of ancient open-ocean DIC. sediment buffered, with those from the correlative upper foreslope (IPz and UFz, respectively; see Fig. 2A for definitions), where carbon isotope excursions | carbonate diagenesis | Neoproterozoic seawater | carbonate platform seawater-buffered diagenesis could have prevailed. As a test of the fidelity of the IPz records, we obtained correlative ones from turbidite fans fringing the platform (LFz and BMz; Fig. 2A), on arbon-isotope records from marine carbonate successions the assumption that they represent epi-platform sediment that aged 870 to 485 Ma (Fig. 1) exhibit large excursions outside C was advected to the foot of the foreslope, potentially beyond the the compositional range of modern seawater DIC (δ13C = 0.8 ± reach of seawater-buffered diagenesis (Fig. 2 B and C). Where the 1.5‰ Pee Dee Belemnite (PDB); ref. 1) or benthic δ13 = ± ‰ foreslope record was truncated by end-Cryogenian (Marinoan) foraminifera ( C 0.9 1.7 VPDB; ref. 2). Neoproterozoic δ13 δ13 − + ‰ glacial erosion, we investigated lateral Ccarb change going sea- Ccarb ranges from 12 to 9 (Vienna Pee Dee Belemnite A (VPDB)), and values of +6.0 ± 2.5‰ are sustained for as long ward across the outer platform (OPz; Fig. 2 ). as ≥50 My (3). It is often assumed that such excursions reflect secular changes in the composition of seawater DIC, because Significance they are quantitatively reproducible regionally (SI Appendix, Fig. S1)(4–8) and some are demonstrably correlative in what were Carbonate sediments of Neoproterozoic age exhibit large geographically remote successions (9, 10). Here we present age- secular excursions of carbon isotope composition outside 13 correlative δ Ccarb records from different parts of a single the range of modern seawater dissolved inorganic carbon Neoproterozoic carbonate platform, the Otavi/Swakop Group (DIC), but their origins are controversial. We show that in a (Fig. 1) in northern Namibia, which collectively support Neoproterozoic carbonate platform in Namibia, such excursions a sedimentary origin for the excursions but imply that they are disappear on the flanks of the platform, where compositions decoupled from open-ocean DIC. are more compatible with modern seawater. We attribute the Our approach was inspired by recent studies of carbonate dia- observed spatial variation to early fluid-buffered alteration on genesis within active marine carbonate platforms utilizing Ca and the flanks of the platform, where seawater invaded the sedi- Mg isotopes (11, 12). In tandem, Ca and Mg isotopes can dis- ment in response to geothermal porewater convection. Ac- criminate between 2 end-member diagenetic regimes, seawater cordingly, the isotope excursions in the platform interior are buffered and sediment buffered. Dolomites produced during decoupled from open-ocean DIC, which remained close to the seawater-buffered diagenesis have δ44Ca values that approach modern range. Our interpretation is testable and, if confirmed, that of seawater (0‰), and relatively uniform δ26Mg values that has important ramifications for the origins of ancient carbon are ∼2‰ lighter than seawater (13). In contrast, dolomites formed isotope excursions. under sediment-buffered conditions have variably elevated δ26Mg values, due to distillation of Mg in the dolomitizing fluid (11). Author contributions: P.F.H. and K.G.L. designed research; P.F.H. and K.G.L. performed Their δ44Ca values are inherited from the precursor carbonate research; K.G.L. analyzed data; and P.F.H. wrote the paper. sediment, ∼1.0‰ (calcite) to ∼1.5‰ (aragonite) lighter than The authors declare no conflict of interest. seawater (14) due to equilibrium fractionation that occurs during This article is a PNAS Direct Submission. primary carbonate production but not during diagenesis (15, 16). Published under the PNAS license. Thus, seawater-buffered dolomite has lighter Mg and heavier Ca 1To whom correspondence may be addressed. Email: [email protected]. than sediment-buffered dolomite (11). This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. Seawater-buffered diagenesis requires that seawater persis- 1073/pnas.1909570116/-/DCSupplemental. tently invades the sediment. This is most likely to occur in Published online September 4, 2019.

18874–18879 | PNAS | September 17, 2019 | vol. 116 | no. 38 www.pnas.org/cgi/doi/10.1073/pnas.1909570116 Downloaded by guest on September 29, 2021 GA to 2.0 km of cyclic shallow-water carbonate of the Subgroup (22, 23). There are no δ13C data from syn-glacial strata because + BIOTIC SM EB SS C 0.6 10 VSM Keele AA

FA’s .. no primary carbonate was produced. The profiles vary appre- Coppercap Huttenberg 0.5 ciably in thickness (SI Appendix,Figs.S3andS4) and were normalized to IPz thickness (Fig. 3) by procedures described +5 SPICE 0.4 below and in SI Appendix. Samples were prepared and isoto- Gaskiers 0.3 pically analyzed according to methods described in refs. 5 (1) (2) and 41. Marinoan (VPDB) 0 0.2

Sturtian Results carb

C 0.1 δ13

13 Cryogenian C profiles from the IPz exhibit 3 negative and 2 -5

Rasthof positive carbon isotope excursions (CIEs) (Fig. 3). For brevity, 0 Shuram we number them Cn1 to 5, with negative CIEs being odd num- Garvellach

Bitter Springs Bitter bered and positive ones even numbered. Cn1 is named Rasthof Maieberg Russoya org Trezona -10 Taishir o (33), Cn3 Taishir (8, 42), and Cn5 Trezona (6, 43). Positive ex- 13 C(i)=-6 /oo o Ccarb Otavi/Swakop Group =27/oo cursion Cn4, named Keele peak (44), is accompanied by in- 13 T ONIAN CRYOGENIAN EDIACARAN CAMBR’N creased point-to-point δ C variability (Fig. 3 and SI Appendix, Fig. S5). 900 850 800 750 700 650 600 550 500 Cryogenian profiles from LFz turbidites generally track the Millions of before present (Ma) IPz (Fig. 3). Cn3 is deeper, but less so than the nominal Taishir Fig. 1. Marine carbonate δ13C from 900 to 485 Ma (3, 66) compiled from CIE in Mongolia (8, 42). Cn4 is shallower than IPz with even various carbonate platforms. Gray bands are ranges for modern seawater more scatter. The descent and nadir of Cn5 went unrecorded

DIC (1) and Cenozoic benthic foraminiferal CaCO3 (2). Named C-isotope ex- in the LFz because of a laterally persistent shale (Narachaams cursions in magenta italics. Vertical blue bars are glacial epochs and aqua Formation). A falling-stand carbonate wedge (Franni-aus For- box indicates total age range of the Otavi/Swakop Group in northern mation) preceding the Marinoan lowstand records the recovery Namibia. Fractional organic burial flux (forg) assumes a C-influx (C(i)) with leg of Cn5 (Fig. 3 and SI Appendix, Fig. S3) in the LFz. It climbs δ13 = − ‰ δ13 – δ13 = ‰ C 6 (VPDB) and Ccarb Corg 27 in the burial flux. Biotic first 1.5‰ higher than the IPz because accumulation continued after appearances: SM, scale microfossils (67); VSM, vase-shaped microfossils (68); the platform was subaerially exposed (35).

GA, algal biomarkers (69); AA, acanthomorphic acritarchs (70); EB, Ediacara EARTH, ATMOSPHERIC,

soft-body fossils (71); SS, small shelly fossils (72); C, fauna (73). The UFz profile tracks the IPz through Cn1 and 2, but Cn3 AND PLANETARY SCIENCES and 4 are not expressed at all (Fig. 3). After Cn2, δ13C stabilizes near 3‰ in the UFz, declining slowly with stratigraphic height. Geologic Setting The top of the profile veers off toward lighter values, which we The southwestern promontory of is blanketed by a tentatively correlate with the Cn5 downturn (Fig. 3). Most of 2 to 4-km-thick carbonate platform that accumulated between Cn5 is missing because Marinoan glacial erosion was focused in δ13 ∼0.77 and 0.59 Ga, and was folded during Ediacaran orogenesis the UFz (35). The C frequency distribution of clasts in the Marinoan glacial deposits (35, 45) implies that Cn5 was pre- (22, 23). The platform had a well-defined southern margin (Fig. ∼ ‰ 2A), beyond which was a distally tapered foreslope and deep- served in the UFz but that its nadir may have been 3 less depleted than the IPz. The Cn5 nadir is also more shallow, by water marine basin. The shallow-water platform succession consti- ∼ ‰ SI Appendix tutes the Otavi Group, while the foreslope and basin facies make up 2.5 , in the OPz relative to the IPz ( , Fig. S5), the Swakop Group (23, 24). The distinction in facies was not per- consistent with a seaward gradient from sediment-buffered to seawater-buffered diagenesis (12). Conversely, the Cn4 positive manently established until ∼654 Ma (basal Ombaatjie Formation; excursion is less enriched, with more point-to-point variability, in Fig. 2A), before which time the area underwent crustal stretching the OPz compared with the IPz (SI Appendix, Fig. S5). In Cn4 but the overall bathymetric zonation had yet to develop (25). and Cn5, OPz values are shifted toward modern norms com- The Sturtian (717 to 659 Ma) and Marinoan (649 to 635 Ma) pared with the IPz (SI Appendix, Fig. S5). panglacial chrons (26–30) left glacial–periglacial deposits and/or “ ” Early Ediacaran profiles in the IPz exhibit CIEs En1 to 4 (Fig. mutually distinctive postglacial cap carbonates that are rec- 3), the last of which predates the middle Ediacaran Shuram CIE ognizable in every zone (Fig. 2A), providing a reliable basis – (46, 47). En1 occurs in all zones, with maximum scatter in the for correlation (25, 31 36). The Otavi/Swakop Group is dis- BMz. The descending leg of En1 (Keilberg cap dolostone) is less conformably overlain by foredeep clastic deposits and both were depleted with more scatter in the foreslope (SI Appendix, Fig. folded during collisional orogeny to the south (Damara Belt) and S4) and OPz (SI Appendix, Fig. S6) than in the IPz, consistent west (Kaoko Belt) starting at 0.59 Ga (37, 38). We relate plat- with seawater-buffered diagenesis (Fig. 2B) (48). Divergence of form destruction to subduction at a trench, initiating collisional δ13C trajectories during En1 recovery (SI Appendix, Fig. S6)is tectonism. attributed to landward progradation during diachronous high- stand (HST) sedimentation (35). Above En1, IPz and foreslope C-Isotope Profiles profiles are decoupled (Fig. 3 and SI Appendix, Fig. S4). The IPz δ13 SI Appendix We selected Ccarb profiles (Fig. 3 and , Fig. S2) and BMz follow parallel trajectories through En2 and 3, but the representing 2 time intervals. The older is the nonglacial Cryogenian, foreslope is 2‰ to 2.5‰ heavier and occupies a range compat- between the panglacial chrons, which is estimated to have lasted 9.8 ible with modern seawater DIC (Fig. 3). The heavier foreslope My,fromcyclostratigraphy(28),or 659 to 649 Ma given radiometric values compared with the IPz were previously interpreted as in- constraints (26, 27, 29, 30). From a CO2 perspective, it represents dicating a “reverse” isotopic gradient in the water column (35), a the Sturtian snowball aftermath (39) and its expression by 0.6 km of postulate obviated by the diagenetic interpretation offered here. carbonate strata in the IPz (Fig. 2A) was made possible by crustal In En4, named Hüttenberg CIE (49) for its host formation stretching and early postrift thermal subsidence (5). The younger (Th; Fig. 3), the IPz is characterized by heavy values with huge time interval (Fig. 3) is early Ediacaran, starting at the terminal (10‰) point-to-point variability (SI Appendix,Fig.S4) (49). Marinoan cap dolostone (40), dated at 635 Ma (26, 27, 30), and Correlative BMz data are also extremely variable but highly de- ending at the foredeep disconformity, which we take to be ≥590 pleted, with values complementary to the IPz but contained in Ma. In the IPz (Fig. 2A), this ≤45 My interval is represented by 1.0 strata that are 10 times thinner (Fig. 2A and SI Appendix, Fig. S4).

Hoffman and Lamothe PNAS | September 17, 2019 | vol. 116 | no. 38 | 18875 Downloaded by guest on September 29, 2021 BMz LFz UFz OPz IPz A South Basin Margin Lower Upper Outer platform Inner platform North S-F Foreslope 500 m .. EDIACARAN Renosterberg Huttenberg smbAbenab Tsumeb 0 50 km Elandshoek O t a v i G r o u p 200x vertical Kuiseb Mulden exaggeration Maieberg (Sesfontein) CRYO.

EDIACARAN R-S Ombaatjie S-F Gruis Huab Rasthof

Chuos Ombombo Abenab Sbgp ridge Makalani Okakuyu 1 Devede Swakop Ugab Sbgp ridge 3 639.29 +- 0.26 Ma Beesvlakte

1 Nosib Naauwpoort 747 + 2 Ma 2 TONIAN C. - 760 + 1 Ma Austerlitz 4 - Nabis 757 +- 5 Ma complex Formation 1 Subgroup 746 + 2 Ma Formation

- Period or Group Group Period Subgroup Chuos Fm (Sturtian glacial) Ghaub Fm (Marinoan glacial) S-F shelf-foredeep transition foredeep clastics (terrestrial) shallow-water carbonate syn-rift clastics R-S rift-to-shelf transition foredeep clastics (marine) deeper-water carbonate syn-rift bimodal volcanics

T(oC) T(oC) B 0 30 Kohout convection 0 30 60 90

seawater

geotherm

sediment- buf fered diagenesis

o T(oC) meteoric diagenesis T( C) C 0 30 0 30 60 90

seawater geotherm

(CaMg)CO

LGM LGM 3 seawater-buf fered diagenesis

Fig. 2. (A) South−north cross-section of the 0.77-0.59-Ga Otavi/Swakop Group (, Namibia) carbonate succession, restored prior to middle 13 Ediacaran folding (25). Note 200× vertical exaggeration. Highlighted are the zones from which δ Ccarb records were selected for this study: inner platform (IPz), outer platform (OPz), upper foreslope (UFz), lower foreslope (LFz), and basin margin (BMz). Differentiation of the platform and basin began at the rift- to-shelf (R−S) transition ∼0.65 Ga. The platform was destroyed by flexure and abortive subduction at the shelf-to-foredeep (S−F) transition ∼0.59 Ga. Sources of radiometric dates: 1 (74), 2 (75), 3 (76), 4 (77). (B) Idealized porewater convection (17, 19, 20) during a sea-level highstand like the Holocene. Flow is driven by the horizontal temperature (density) gradient between seawater and porewater in the platform interior. Fluid-buffered diagenesis is predicted to occur on the foreslope, where seawater continually invades the sediment. Rock-buffered diagenesis occurs farther along the porewater pathway (11, 12). (C) Same as in B but during a sea-level lowstand like the (LGM) when the platform top was emergent and therefore a recharge area for meteoric groundwater (18, 21) or brine reflux (78, 79).

In the foreslope zone, δ13C remains within the range of modern This conclusion echoes the positive “isotope conglomerate tests” seawater to the top of the profile, except for a short-lived En4 of the Shuram CIE (Fig. 1) in South Australia (51). 13 (Fig. 3). Average foreslope values are ≤9‰ lighter and simulta- The simplest explanation for the relatively stable δ C values neously ≤8‰ heavier than stratigraphic equivalents in the IPz in the UFz is that they were diagenetically altered toward values ∼ ‰ ∼ ‰ and BMz, respectively. The expanded area of alteration in the that slowly fell from 3.5 (VPDB) in Cn3 to 0.5 at the + end of En4 (Fig. 3). According to the simple diagenetic model early Ediacaran (LFz UFz) compared with the Cryogenian (UFz B C only) is attributable to vertical growth of the platform and modest described above (Fig. 2 and ), early diagenesis and dolomi- tization in the FSz was more seawater buffered while that in the progradation (Fig. 2A). IPz was more sediment buffered. The net effect of seawater- Crossplots of δ13C and δ18O post-En1 in each zone are shown buffered diagenesis in the UFz was to destroy CIEs, not to in SI Appendix, Fig. S7. The 2 are positively correlated in the IPz, δ18 ∼ ‰ create them. where the Orangeis 4 ,butweaklycorrelatedinthe Cn1 and 2 are preserved in the UFz because the foreslope did δ18 ‰ foreslope zone, where O ranges over 10 . not develop until after the rift-to-shelf (R−S) transition at the base of Cn4 (Fig. 3). Seawater-buffered diagenesis appears to Discussion begin in the UFz during Cn3, prior to this transition. This could – δ13 Why do the LFz BMz C profiles (Fig. 3) track the IPz ones simply reflect different ages of deposition and diagenesis. When more closely than do the UFz profiles (Fig. 3)? We suggest that the foreslope first developed, Cn3-age sediment was still at shal- the fringing turbidites are composed of carbonate that was shed low burial depths and subject to seawater-buffered diagenesis. 13 off the platform with its sedimentary δ C intact (e.g., ref. 50). En1 also occurs in the UFz (Fig. 3), but the Keilberg We infer that they accumulated beyond the zone of seawater- Member is less 13C-depleted in the OPz than in the IPz (52) buffered diagenesis (Fig. 2 B and C). If correct, these inferences (SI Appendix,Fig.S6). Cn5, which is not preserved in the UFz imply that the IPz CIEs are not products of burial diagenesis. (Fig. 3), is also less 13C-depleted in the OPz than in the IPz

18876 | www.pnas.org/cgi/doi/10.1073/pnas.1909570116 Hoffman and Lamothe Downloaded by guest on September 29, 2021 Age carbonate rock. But the implications for the origins of CIEs pre- KAOKO-DAMARA FOREDEEP CIE served in areas of sediment-buffered diagenesis are troubling. If 590 1.4 the CIEs are decoupled from DIC in the well-mixed ocean, why 1.2 are they reproducible regionally (SI Appendix, Figs. S1 and S5)? Th 1.0 Why is there growing evidence for synchroneity between CIEs on distant platforms (9, 10, 46, 59, 60)? Is there a mystery process that 0.8 En4 generates CIEs zonally, if not globally, independent of open-ocean DIC? Such a process should no longer be in operation (Fig. 1). 0.6 IPz Te En3 We should not dodge these questions for want of satisfactory 0.4 UFz answers. In SI Appendix, we speculate that nucleation kinetics are LFz-BMz 0.2 En2 involved in CIE generation in epi-platform waters at times of intense Tm Maieberg En1 photosynthesis or evaporation. Large point-to-point variability dur- 635 0 km MARINOAN SNOWBALL ing positive CIEs (Fig. 3) is more suggestive of isotopic reservoir effects than equilibrium fractionation in a well-mixed ocean. Our 649 0.6 Trezona Cn5 speculation is also prompted by an apparent correlation between Ab SI Appendix 0.4 Cn4 periods when CIEs were large ( ,Fig.S8), the R-S Ag Taishir Cn3 carbonate factory was producing more mud than sea-floor cement (61), and aerobic respiration created steeper saturation gradients 0.2 (1) Ar (2) Cn2 with respect to CaCO3 in the water column (62). The resulting Cryogenian early Ediacaran SI Appendix Rasthof Cn1 pattern of secular change ( ,Fig.S8)isbroadlyconsistent 659 0 with that inferred from multi-isotope (δ34S, Δ33S, δ18O, and Δ17O) km STURTIAN SNOWBALL investigations of marine sulfate over geologic time (63). Ma -5 0 5 10 Is it possible that the spatial δ13C patterns we observe (Fig. 3) 13 o C carb ( /oo VPDB) R-S: rift-to-shelf are simply an artifact of small-scale miscorrelations within the two time boxes? The most uncertain correlation is the UFz record δ13 Fig. 3. Comparison of depositional age-correlative Ccarb profiles from equivalent to Cn4 (SI Appendix,Fig.S3). The selected section the inner platform (blue), upper foreslope (purple), and combined lower (P8513 Duurwater, SI Appendix,Fig.S2)istoptruncatedby foreslope and basin margin (orange) spanning the nonglacial (inter-Snowball) Marinoan glacial erosion, but the last 5 data points veer off to 4‰ Cryogenian and early Ediacaran of the Otavi/Swakop Group, Namibia (Fig. δ13 EARTH, ATMOSPHERIC, lighter C values that we correlate with the start of Cn5 (Fig. 3). AND PLANETARY SCIENCES 2A). Gray bands as in Fig. 1. For profile locations see SI Appendix, Fig. S2 We do not relate this isotopic shift to glaciation, because 4 parallel and Dataset S1. Some locations changed from lower foreslope in Cryogenian to upper foreslope in Ediacaran time due to foreslope progradation. The sections (over a 15-km strike length) with high-resolution data profiles were normalized to the stratigraphic height of the IPz section (blue, showing the same uniform values lack any shift under the glacial in kilometers) at the dashed blue lines separating IPz formations: Ar, Rasthof Fm; surface. This is easily attributed to glacial erosion of a preglacial Ag, Gruis Fm; Ab, Ombaatjie Fm; Tm, Maieberg Fm; Te, Elandshoek Fm; CIE. If the correlation with Cn5 is rejected, the inferred duration Th, Hüttenberg Fm. The platform aggraded in response to thermal sub- of near-uniform Cryogenian values in the UFz could be reduced, sidence after the R−S transition, whereas the basin was drowned. Conse- possibly even negating their correlation with Cn4 (Fig. 3). How- quently, the foreslope developed progressively after Ag deposition. Note ever, the interval of uniform values is 685 m in stratigraphic thick- + that the LFz BMz profiles track the IPz through 5 negative and 3 positive ness (SI Appendix, section P8513, Dataset S1, sheet 2), meaning that − CIEs until En4, but that the UFz record is stable close to the range of modern average sedimentation rates (≥0.1 mm y 1) were high even with the seawater DIC (gray band; ref. 1) through Cn3 to 4 and En2 to 4. The UFz does SI Appendix record the En1 CIE and likely the Cn5 CIE, since δ13C values ∼3‰ have a duration stretched as indicated (Fig. 3 and ,Fig.S3). minimum frequency in Marinoan glacial debris, despite subglacial erosion of The interpretations offered here are unsettling but make predic- 13 upper Ab in the UFz (34, 35, 45). We infer that the δ Ccarb was altered tions that should be vigorously tested with carbon isotopes in other during fluid-buffered diagenesis in the UFz (Fig. 2B) after Cn2 and that the Neoproterozoic−Cambrian carbonate platforms where foreslope altered values best represent the composition of open-ocean DIC at the time facies are preserved. The Cryogenian Balcanoona Formation of of diagenesis. Accordingly, the CIEs observed in the IPz, and which were South Australia (64) and the early Ediacaran Doushantuo/Lantian advected as sediment to the LFz+BMz, are indigenous to epi-platform waters. Formation of South China (65) are prominent and accessible among numerous suitable targets. Our interpretations also make testable predictions for Mg and Ca isotopes as described above—dolomites (SI Appendix,Fig.S5). This could reflect less extreme Cn5 and 26 44 in the UFz should have lower δ Mg and higher δ Ca than their En1 CIEs in open-ocean DIC compared with epi-platform waters. IPz correlatives (11, 12). The same trends should be found in a Alternatively, it could indicate incomplete alteration during seaward direction across the platform. To date, these tests have seawater-buffered diagenesis of Cn5 and En1. been conducted in the Otavi/Swakop Group only in the Keilberg Cn5 may be incompletely altered because seawater-buffered Member of En1 (48) (SI Appendix,Fig.S6). Mg and Ca isotope data diagenesis was suspended during Marinoan glaciation. The hy- from a selection of our samples will be a truly independent test of draulic head created by an ice sheet over the platform, com- the inference presented here since such data do not yet exist. pounded by the lowering of sea level (53, 54), should have reversed the direction of porewater flow, causing sediment-buffered dia- Conclusions genesis in the UFz and OPz during Marinoan time. En1 may be The upper foreslope of the Neoproterozoic Otavi/Swakop car- 13 incompletely altered because of high sediment accumulation and bonate platform has stable δ Ccarb values compatible with lithification rates in the Marinoan aftermath, when anomalous modern seawater DIC, while correlative profiles from the inner carbonate oversaturation was maintained by ocean warming, shelf platform and from allodapic fans basinward of the foreslope flooding, alkalinity input, and pH rise (39, 55–58). exhibit long-lived positive and negative excursions that appear to If the prevailing UFz δ13C values of 1.5 ± 2.5‰ (VPDB) replicate CIEs in other carbonate platforms. We suggest that the represent complete re-equilibration during seawater-buffered dia- upper foreslope, and to a lesser degree the outer platform, un- 13 genesis, then δ CDIC in the nonglacial Cryogenian and early derwent early seawater-buffered diagenesis, driven by geothermal Ediacaran ocean was barely distinguishable from modern (0.8 ± porewater convection. In contrast, diagenesis in the platform in- ‰ 13 1.5 PDB; ref. 1). In itself, this is unimportant for the carbon terior was sediment buffered and retained δ Ccarb values, as did geochemical cycle, since DIC is a small reservoir compared to sediment advected off the platform beyond the reach of seawater

Hoffman and Lamothe PNAS | September 17, 2019 | vol. 116 | no. 38 | 18877 Downloaded by guest on September 29, 2021 invasion. The suggested pattern of diagenesis is testable with tandem ACKNOWLEDGMENTS. We are grateful to Dan Schrag and Galen Halverson Ca and Mg isotopes. If confirmed, it will imply that during a period for carbon isotope measurements performed in the Geochemical δ13 Oceanography Laboratory at Harvard University and in the Department when platform carbonate is characterized by large Ccarb excursions, of Earth and Planetary Sciences at McGill University, respectively. We open-ocean DIC was stable within the modern range of values. We thank Anne-Sofie Ahm and Peter Crockford for comments on earlier speculate that nucleation kinetics associated with nonskeletal carbon- drafts, and Francis Macdonald for encouragement and suggestions on ate production in the water column, a process no longer in operation, intrabasinal correlations. We thank 2 anonymous reviewers for construc- caused the C isotope composition of DIC in epi-platform waters to tive suggestions, and Tony Prave for information regarding Rhyacian carbonates. We thank all those who assisted in the field over many years. fluctuate widely and quasi-synchronously in far-flung but possibly Field work was authorized and supported by the Geological Survey of cozonal areas, since this behavior is limited to more oxygenated Pro- Namibia. We thank owners and residents in the Kunene Region for access terozoic periods, when surface waters were most highly oversaturated. to their land.

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