Reconstructing carbonate alteration histories in orogenic sedimentary basins: Xigaze forearc, southern Tibet

Carbonate clumped isotope thermometry (T(Δ_(47))) and the oxygen stable isotope record (δ^(18)O) provide critical constraints on the temperature of carbonate mineral formation and isotopic composition of ancient waters used in reconstructions of past climate, tectonics, and ecological and environmental change. The robust use of these proxies requires that carbonate minerals in the rock record retain primary isotopic compositions through often complex post-depositional thermal histories. New carbonate clumped isotope thermometry and sedimentology data are paired with existing thermochronology and isotope exchange reaction modeling to provide unique constraints on the alteration history of the carbon and oxygen isotopes in Tethyan marine carbonates (Jialazi Fm, Tso Jianding Group) of the Xigaze forearc, southern Tibet. Within the framework of the textural and isotopic data, the effects of early and late burial alteration on Δ_(47) and δ^(18)O values of marine rocks that were buried to 4 km above sea level are modeled. The alteration products are considered within two well-studied mechanistic frameworks: water-rock recrystallization and solid-state reordering of ^(13)C-^(18)O bonds at high temperature. The proposed alteration model is as follows: (1) shallow, early diagenesis with water of similar composition and temperature to formation waters infills primary porespace with microspar; (2) high-temperature, rock-buffered water-rock exchange and partial solid-state reordering at >3-4 km depth; and (3) near-surface, low-temperature water-rock exchange on the exhumation pathway. The Δ_(47) heterogeneity within individual samples (0.456 to 0.721) requires significant differential alteration on the retrograde path (high to low temperature, low water-rock ratio) in which primary compositions and compositions inherited during early burial diagenesis are overprinted with modern meteoric values. A low T(Δ_(47)) value is often assumed to record the primary temperature of the depositional environment. However, samples yielding low T(Δ_(47)) values in the Jialazi Fm have undergone extensive high-temperature water-rock alteration, and therefore, require a mechanism for increasing ^(13)C-^(18)O bond ordering on the retrograde pathway while preserving many primary biogenic structures and micritic textures. This study highlights the potential significance of late-stage, low-temperature alteration of carbonates derived from orogenic sedimentary basins for studies of terrestrial paleoenvironments and tectonics. This study also provides an example framework for combining observational and analytical data to reconstruct a carbonate alteration history of a buried sedimentary package

Reconstructing carbonate alteration histories in orogenic sedimentary basins: Xigaze forearc, southern Tibet

Carbonate clumped isotope thermometry (T(Δ_(47))) and the oxygen stable isotope record (δ^(18)O) provide critical constraints on the temperature of carbonate mineral formation and isotopic composition of ancient waters used in reconstructions of past climate, tectonics, and ecological and environmental change. The robust use of these proxies requires that carbonate minerals in the rock record retain primary isotopic compositions through often complex post-depositional thermal histories. New carbonate clumped isotope thermometry and sedimentology data are paired with existing thermochronology and isotope exchange reaction modeling to provide unique constraints on the alteration history of the carbon and oxygen isotopes in Tethyan marine carbonates (Jialazi Fm, Tso Jianding Group) of the Xigaze forearc, southern Tibet. Within the framework of the textural and isotopic data, the effects of early and late burial alteration on Δ_(47) and δ^(18)O values of marine rocks that were buried to 4 km above sea level are modeled. The alteration products are considered within two well-studied mechanistic frameworks: water-rock recrystallization and solid-state reordering of ^(13)C-^(18)O bonds at high temperature. The proposed alteration model is as follows: (1) shallow, early diagenesis with water of similar composition and temperature to formation waters infills primary porespace with microspar; (2) high-temperature, rock-buffered water-rock exchange and partial solid-state reordering at >3-4 km depth; and (3) near-surface, low-temperature water-rock exchange on the exhumation pathway. The Δ_(47) heterogeneity within individual samples (0.456 to 0.721) requires significant differential alteration on the retrograde path (high to low temperature, low water-rock ratio) in which primary compositions and compositions inherited during early burial diagenesis are overprinted with modern meteoric values. A low T(Δ_(47)) value is often assumed to record the primary temperature of the depositional environment. However, samples yielding low T(Δ_(47)) values in the Jialazi Fm have undergone extensive high-temperature water-rock alteration, and therefore, require a mechanism for increasing ^(13)C-^(18)O bond ordering on the retrograde pathway while preserving many primary biogenic structures and micritic textures. This study highlights the potential significance of late-stage, low-temperature alteration of carbonates derived from orogenic sedimentary basins for studies of terrestrial paleoenvironments and tectonics. This study also provides an example framework for combining observational and analytical data to reconstruct a carbonate alteration history of a buried sedimentary package

Paleocene to Pliocene low-latitude, high-elevation basins of southern Tibet: implications for tectonic models of India-Asia collision, Cenozoic climate, and geochemical weathering

The elevation history of the Tibetan Plateau promises insight into the mechanisms and dynamics that develop and sustain high topography over tens of millions of years. We present the first nearly continuous Cenozoic elevation history from two sedimentary basins on the southern Tibetan Plateau within the latest Cretaceous to Eocene Gangdese arc. Oxygen-isotope and Δ47 clumped-isotope compositions of nonmarine carbonates allow us to constrain carbonate formation temperature and reconstruct the paleoprecipitation record of the Eocene to Pliocene Oiyug Basin and Paleocene to Eocene Penbo Basin. We exploit the systematic decrease of surface temperature and meteoric water δ18O values with elevation to derive paleoelevation estimates for these basins. Minimally altered and unaltered pedogenic and lacustrine carbonates from the Oiyug Basin yield Δ47, CDES (relative to the carbon dioxide equilibrium scale [CDES]) values of 0.625‰ to 0.755‰, which correspond to temperatures of 1−30 °C using a Δ47 thermometer for low-temperature carbonates. Similarly, the Penbo Basin yielded Δ47, CDES values of 0.701‰ to 0.726‰, corresponding to temperatures of 6−12 °C. The apparent evidence for survival of primary clumped-isotope values in the face of substantial burial and heating is an important result for the field of carbonate clumped-isotope thermometry. Our paleoelevation estimates for the Eocene to Pliocene Oiyug Basin (∼6.5−4.1 km) support previous evidence that high elevations were attained in southern Tibet by at least ca. 30 Ma. Stable-isotope results allow for the possibility of significant topographic subsidence during the Miocene as a result of regional extension. In the Penbo Basin, our paleoelevation estimates for the Paleocene to Eocene Nianbo Formation (4.4 +1.3/−1.7 km) and Eocene Pana Formation (4.1 +1.2/−1.6 km) extend the altitude record of the southern Tibetan Plateau to pre−India-Asia collision. These results support the “Lhasaplano” model of an Andean-type continental margin tectonic system. The rise of the Himalayas and Tibet is often invoked to understand isotopic proxies for global chemical weathering in the Cenozoic and has constrained the debate on the nature of CO2−climate−weathering feedbacks. The nature of the Tibetan paleoelevations from pre- to postcollision, as presented here, indicates that high relief at low latitude prevailed on the Asian margin much earlier than previously thought. Thus, high topography alone at low latitude is not sufficient to account for the Cenozoic weathering proxy record

Leveraging the carbonate record from regional hydroclimate to microbial ecology

Org. and inorg. stable isotopes of lacustrine carbonate sediments are commonly used in reconstructions of ancient terrestrial environments. However, different carbonate fabrics can form contemporaneously by localized processes within a single lake. Microbes and hydrol. alter the chem. of their local environments, which can overprint the broader environmental signal preserved in the carbonate record. In this work, we explore the susceptibility of different lacustrine carbonate facies to early diagenetic and in situmicrobial processes. To do so, we characterize geochem. and stable isotopic variability of carbonate minerals, org. matter, and water within one modern lake (Great Salt Lake, UT) within the context of seasonal and site-specific 16S rRNA amplicon sequencing community profiles. We find that facies equiv. to ooid grainstones provide time-averaged records of lake chem. that are resistant to alteration by microbial activity, whereas microbialite, intraclasts, and carbonate mud are more susceptible to local microbial influence and hydrol. Further, we find localized occurrences of clumpedisotopic disequil. in the near subsurface likely driven by local microbial metab. (e.g. sulfur cyclers) during authigenic carbonate pptn. Our findings provide a framework for leveraging carbonate facies-specific C, O, and clumped isotope records in ancient lakes to reconstruct both regional hydroclimate and local microbial ecol

Reconsidering the uplift history and peneplanation of the northern Lhasa terrane, Tibet

The elevation history of the Tibetan Plateau promises insight into the mechanisms and dynamics that develop and sustain high topography over tens of millions of years, as well as the contribution of uplift-related erosive flux to Cenozoic global cooling. The elevation history of the center and northern margin of the plateau have been historically less well-constrained than the southern margin. A diverse suite of techniques, each with their own biases and uncertainties, yield discrepant mid-Cenozoic elevation estimates (0–5 km). We reconstruct Paleogene to Miocene elevations of the Lunpola basin on the northern Lhasa terrane, the southernmost crustal block of the Tibetan Plateau, using stable isotope paleoaltimetry and clumped-isotope paleothermometry on lacustrine and pedogenic carbonates, integrated with previously published compound-specific n-alkane-derived hydrogen isotopes. Paleo-elevation estimates for the Lunpola basin (∼3.1–4.7 km) demonstrate that the northern edge of the Lhasa Block attained high elevation prior to ∼24 Ma and potentially by the Early Eocene (<48 Ma). Our results allow for the possibility that the entire Lhasa Block was composed of extremely thick continental crust at the initiation of India-Asia collision, rather than restricting the extent of thick crust to the Linzizong volcanic arc (“Lhasaplano” model) or Gangdese Mountains, and also refutes studies invoking a low elevation interpretation based on higher oxygen and compound-specific δD isotope values. Better constraints on depositional ages in the Lunpola basin are needed to refine the early Cenozoic elevation history along the Bangong-Nujiang suture zone. Finally, we posit that interpretations of proxy data can be biased by incomplete or selective sampling, and propose multi-proxy, intrinsically cross-disciplinary studies to resolve inconsistent interpretations from otherwise unrelated proxies

Reconsidering the uplift history and peneplanation of the northern Lhasa terrane, Tibet

The elevation history of the Tibetan Plateau promises insight into the mechanisms and dynamics that develop and sustain high topography over tens of millions of years, as well as the contribution of uplift-related erosive flux to Cenozoic global cooling. The elevation history of the center and northern margin of the plateau have been historically less well-constrained than the southern margin. A diverse suite of techniques, each with their own biases and uncertainties, yield discrepant mid-Cenozoic elevation estimates (0–5 km). We reconstruct Paleogene to Miocene elevations of the Lunpola basin on the northern Lhasa terrane, the southernmost crustal block of the Tibetan Plateau, using stable isotope paleoaltimetry and clumped-isotope paleothermometry on lacustrine and pedogenic carbonates, integrated with previously published compound-specific n-alkane-derived hydrogen isotopes. Paleo-elevation estimates for the Lunpola basin (∼3.1–4.7 km) demonstrate that the northern edge of the Lhasa Block attained high elevation prior to ∼24 Ma and potentially by the Early Eocene (<48 Ma). Our results allow for the possibility that the entire Lhasa Block was composed of extremely thick continental crust at the initiation of India-Asia collision, rather than restricting the extent of thick crust to the Linzizong volcanic arc (“Lhasaplano” model) or Gangdese Mountains, and also refutes studies invoking a low elevation interpretation based on higher oxygen and compound-specific δD isotope values. Better constraints on depositional ages in the Lunpola basin are needed to refine the early Cenozoic elevation history along the Bangong-Nujiang suture zone. Finally, we posit that interpretations of proxy data can be biased by incomplete or selective sampling, and propose multi-proxy, intrinsically cross-disciplinary studies to resolve inconsistent interpretations from otherwise unrelated proxies

Carbonate facies-specific stable isotope data record climate, hydrology, and microbial communities in Great Salt Lake, UT

Organic and inorganic stable isotopes of lacustrine carbonate sediments are commonly used in reconstructions of ancient terrestrial ecosystems and environments. Microbial activity and local hydrological inputs can alter porewater chemistry (e.g., pH, alkalinity) and isotopic composition (e.g., δ¹⁸O_(water), δ¹³C_(DIC)), which in turn has the potential to impact the stable isotopic compositions recorded and preserved in lithified carbonate. The fingerprint these syngenetic processes have on lacustrine carbonate facies is yet unknown, however, and thus, reconstructions based on stable isotopes may misinterpret diagenetic records as broader climate signals. Here, we characterize geochemical and stable isotopic variability of carbonate minerals, organic matter, and water within one modern lake that has known microbial influences (e.g., microbial mats and microbialite carbonate) and combine these data with the context provided by 16S rRNA amplicon sequencing community profiles. Specifically, we measure oxygen, carbon, and clumped isotopic compositions of carbonate sediments (δ¹⁸Ocarb, δ¹³C_(carb), ∆₄₇), as well as carbon isotopic compositions of bulk organic matter (δ¹³C_(org)) and dissolved inorganic carbon (DIC; δ¹³C_(DIC)) of lake and porewater in Great Salt Lake, Utah from five sites and three seasons. We find that facies equivalent to ooid grainstones provide time‐averaged records of lake chemistry that reflect minimal alteration by microbial activity, whereas microbialite, intraclasts, and carbonate mud show greater alteration by local microbial influence and hydrology. Further, we find at least one occurrence of ∆₄₇ isotopic disequilibrium likely driven by local microbial metabolism during authigenic carbonate precipitation. The remainder of the carbonate materials (primarily ooids, grain coatings, mud, and intraclasts) yield clumped isotope temperatures (T(∆₄₇)), δ¹⁸O_(carb), and calculated δ¹⁸O_(water) in isotopic equilibrium with ambient water and temperature at the time and site of carbonate precipitation. Our findings suggest that it is possible and necessary to leverage diverse carbonate facies across one sedimentary horizon to reconstruct regional hydroclimate and evaporation–precipitation balance, as well as identify microbially mediated carbonate formation