Lipid biomarker and stable isotopic profiles through Early-Middle Ordovician carbonates from Spitsbergen, Norway

Abstract

One of the most dramatic episodes of sustained diversification of marine ecosystems in Earth history took place during the Early to Middle Ordovician Period. Changes in climate, oceanographic conditions, and trophic structure are hypothesised to have been major drivers of these biotic events, but relatively little is known about the composition and stability of marine microbial communities controlling biogeochemical cycles at the base of the food chain. This study examines well-preserved, carbonate-rich strata spanning the Tremadocian through Upper Dapingian stages from the Oslobreen Group in Spitsbergen, Norway. Abundant bacterial lipid markers (elevated hopane/sterane ratios, average = 4.8; maximum of 13.1), detection of Chlorobi markers in organic-rich strata, and bulk nitrogen isotopes (δ 15 N total ) averaging 0 to -1‰ for the open marine facies, suggest episodes of water column redox-stratification and that primary production was likely limited by fixed nitrogen availability in the photic zone. Near absence of the C 30 sterane marine algal biomarker, 24-n-propylcholestane (24-npc), in most samples supports and extends the previously observed hiatus of 24-npc in Early Paleozoic (Late Cambrian to Early Silurian) marine environments. Very high abundances of 3β-methylhopanes (average = 9.9%; maximum of 16.8%), extends this biomarker characteristic to Early Ordovician strata for the first time and may reflect enhanced and sustained marine methane cycling during this interval of fluctuating climatic and low sulfate marine conditions. Olenid trilobite fossils are prominent in strata deposited during an interval of marine transgression with biomarker evidence for episodic euxinia/anoxia extending into the photic zone of the water column.

A new constraint on the antiquity of ancient haloalkaliphilic green algae that flourished in a ca. 300 Ma Paleozoic lake

“…Biomarker distributions are unusual compared to typical Paleozoic marine assemblages (Haddad et al., 2016; Lee et al., 2019; Martinez et al., 2019; Rohrssen et al., 2013), but some key characteristic features are consistent with those reported previously from ancient salinity‐stratified lakes of younger ages (Figures 3 and 4; Table 2). These include (a) at least episodic stratification characterized by the moderate–high gammacerane/C 30 αβ hopane values (mean = 0.26, up to 0.65) (Tulipani et al., 2015), (b) no detectable signal (any apparent traces of C 30 αααR sterane (<0.1%) likely represents interference peaks or “cross‐talk” from C 30 methylsteranes rather than genuine C 30 regular sterane signal) of 24‐ n ‐propylcholestanes from marine pelagophyte algae (Figure 5), thus providing strong indication of a lacustrine paleoenvironment for all investigated strata, (c) anomalously high amounts of preserved C 40 carotanes relative to n ‐alkanes (mainly β‐carotane plus lower abundances of γ‐carotane), (d) elevated 3β‐MeHI values up to 14.2% (mean = 6.5%) pointing to an enhanced methane cycle within the sediments and water column (Rohrssen et al., 2013), and (e) low pristane/phytane ratios (<1.0; mean = 0.6) characteristic of bottom water reducing conditions, as well as small but discernible peaks for C 21 –C 25 regular isoprenoids relative to n ‐alkanes likely reflecting from halophilic archaeal communities (Figure 3a; Grice, Schouten, Nissenbaum, et al, 1998; McKirdy et al., 1984).…”

“…Three samples were obtained from the deeper water F 1 member. Rock bitumens were extracted with organic solvents and fractionated using established ancient biomarker methodology (Haddad et al., 2016; Lee et al., 2019; Martinez et al., 2019; Rohrssen et al., 2013) to minimize contamination artifacts. The systematic behavior of maturity‐sensitive alkane ratios from core to core is consistent with these being endogenous and genuine Paleozoic biomarker assemblages (Figure 2).…”

“…Green algae were once the most prolific phytoplankton in Ediacaran and Paleozoic oceans but now exhibit greater abundance and diversity within fresh waters since they were outcompeted by other marine algal groups as major primary producers beginning in the late Paleozoic Era (Falkowski et al., 2004; Knoll, 1992). It is well established that green algae and land plants progressively colonized freshwater aquatic and terrestrial habitats through the Paleozoic Era (Falkowski et al., 2004; Feist et al., 2005; Gray, 1993; Lee et al., 2019; Tulipani et al., 2015), though much less is known about the ecology and major primary producers of Paleozoic saline and alkaline lakes as aquatic habitats. The pH tolerances of modern green algae have been studied previously from both natural environments and laboratory cultures.…”

A Decrease to Low Carbonate Clumped Isotope Temperatures in Cryogenian Strata

“…Though empirical, such alteration indices are thought to indicate lower burial temperatures (Epstein et al, 1977). Limited thermal alteration is also evident from intact biomarkers of the Oslobreen Group strata, consistent with midoil window thermal maturity (RockEval Pyrolysis T max values of 441–446°C; Lee et al, 2019). Midoil window constraints on burial temperatures (<150°C; e.g., Peters et al, 2005) would require burial depths less than or comparable to the 4–6 km of modeled exhumation of regional Caledonian granites since 80–180 Ma (Dörr et al, 2012; Dörr et al, 2019) from apatite fission track analysis.…”

Coincidence of photic zone euxinia and impoverishment of arthropods in the aftermath of the Frasnian-Famennian biotic crisis

“…Moreover, isorenieratane closely correlates with the 2,3,6−/3,4,5-diaryl isoprenoid, i.e., the so called palaeorenieratane (R 2 = 0.95), which usually co-occurs with isorenieratane in Palaeozoic sedimentary rocks (e.g. 4,42–49 ). Additionally, there is a strong correlation with the concentration of aryl isoprenoids, which are compounds that are degradation products of isorenieratane (e.g.…”