The effects of diagenesis on lithium isotope ratios of shallow marine carbonates

Abstract

In this study, we present new data on the δ 7 Li values and Li/(Ca+Mg) ratios of carbonate cores from the Great Bahama Bank (Clino, Unda), a deep water core off of the bank top (ODP Leg 166 Site 1007), and the coralline Key Largo Limestone. We use these samples to evaluate the influence of meteoric diagenesis, marine burial diagenesis, and dolomitization on the Li isotope system in carbonates. We find that recrystallization of aragonite to low-Mg calcite in the presence of meteoric fluids results in a systematic decrease of the Li/(Ca+Mg) ratio in Clino, Unda and Key Largo samples, due to the lower Li/(Ca+Mg) ratio in meteoric fluids compared to seawater. For Li isotopes, we observe that the δ 7 Li of meteorically altered low-Mg calcite is +22.0±3.8‰ (n=28, 1σ), which is coincidentally similar to the original aragonite-rich sediments (+22±1‰ in the Bahamas, +18±1‰ in Key Largo), but with a larger variability (from +15 to +27‰). We interpret these features as reflecting the overprinting of primary Li during meteoric alteration with a highly variable isotope signature that may be controlled by a combination of local porewater and/or global climatic conditions; in either case, meteoric diagenesis produces isotopic signatures that are unrelated to seawater composition. In contrast, marine burial diagenesis and dolomitization of Clino and Unda sediments under "fluid-buffered" conditions result in Li isotope composition that is similar (+30.2±1.5‰, n=36, 1σ) to modern seawater (+31‰). For Site 1007, the δ 7 Li values range between +23‰ and +31‰. We interpret this range as reflecting a combination of varying diagenesis style (fluid to sediment-buffered) and varying contribution of calcite derived from pelagic sediments, with distinct isotopic composition due to primary mineralogy. Altogether, our results show that diagenesis does not invalidate the use of bulk carbonates for deriving Li isotope paleo-records, but the reliability of past carbonates as recorders of seawater δ 7 Li values will depend on carefully characterizing their diagenetic history.

Lithium isotope evidence for enhanced weathering and erosion during the Paleocene-Eocene Thermal Maximum

“…Our carbonate records suggest a pre-PETM seawater  7 Li value of ~26.6 ± 1.3‰ [where the uncertainty stems from uncertainty in the fractionation factor into carbonate (7)], in agreement with the longterm fractionation-corrected Cenozoic Li isotope record (24,29), and a peak LIE  7 Li value of 23 to 24‰ during the PETM. While there is no known temperature effect on Li isotope fractionation during bulk calcite mineralization (6,7,(30)(31)(32), there is a temperature effect on the fractionation factor imposed during Li uptake into secondary minerals (36).…”

“…S6) (7) and separated foraminifera, we are confident that these records are reliably recording paleo-seawater  7 Li values. When compared to the two ODP bulk carbonate sections, the slightly higher absolute  7 Li values recorded by the foraminifera at site 1210 are consistent with a slightly lower isotopic fractionation factor in foraminifera compared to bulk carbonate (29). Therefore, identical seawater values are reconstructed from all sites, with  7 Li seawater ~ 26.4 to 26.8‰ before the PETM, which agrees with the fractionationcorrected Cenozoic foraminiferal record (24).…”

“…3, C and G) and separately by the carbonate fractionation factor (Fig. 3, D and H) (7,29). The latter larger uncertainty has been propagated backward to the riverine flux and  7 Li values (Fig.…”

Lithium Isotopes

“…Modern bulk aragonites have a fractionation factor of Δ 7 Li aragonite-seawater~-10‰ (Gabitov et al, 2011;Marriott et al, 2004b;Pogge von Strandmann et al, 2019b), in agreement with inorganic aragonites (Marriott et al, 2004b;Pogge von Strandmann et al, 2019b). Similarly, biogenic aragonites such as corals (mean Δ 7 Li aragonite-seawater = -12±2‰, 1σ, (Bastian et al, 2018;Marriott et al, 2004a;Marriott et al, 2004b;Rollion-Bard et al, 2009) and aragonitic mollusks (mean Δ 7 Li aragonite-seawater = -13±3‰, 1σ (Bastian et al, 2018;Dellinger et al, 2020)) have Δ 7 Li similar to inorganic aragonite (~-10‰). Calcites tend to have lower fractionation factors than aragonite, with Δ 7 Li calcite-seawater~-6‰ in modern core tops.…”

Geochemical fingerprints of dolomitization in Bahamian carbonates: Evidence from sulphur, calcium, magnesium and clumped isotopes

“…During the Pleistocene, the upper portion of the sedimentary succession was periodically exposed to meteoric diagenesis due to glacio‐eustatic sea‐level changes, while older sediments have only been affected by marine processes (Swart & Oehlert, 2019). Because modern sediments in the Bahamas are dominated by non‐skeletal carbonates (Beach & Ginsburg, 1980; Reijmer et al ., 2009), interest in their geochemistry and diagenesis is widespread among workers studying Earth’s early history (Swart & Kennedy, 2012; Hardisty et al ., 2017; Ahm et al ., 2018; Chen et al ., 2018; Higgins et al ., 2018; Tissot et al ., 2018; Dellinger et al ., 2019; Liu et al ., 2019). In addition, the presence of widespread dolomites in the shallow subsurface of the Bahamas provides an opportunity to study this important, but poorly understood diagenetic process, where the timing and biogeochemical processes that drive dolomitization can be more precisely characterized.…”