Pacific 187 Os/ 188 Os isotope chemistry and U–Pb geochronology: Synchroneity of global Os isotope change across OAE 2

Abstract

Studies of OAE 2 sections beyond the Atlantic Ocean, Western Interior Seaway (WIS) and European pelagic shelf are limited. Here, we present initial osmium isotope stratigraphy ( 187 Os/ 188 Os-Os i ) from two proto-Pacific sites that span the Cenomanian-Turonian boundary interval (CTBI): the Yezo Group (YG) section, Hokkaido, Japan, and the Great Valley Sequence (GVS), California, USA; to evaluate the 187 Os/ 188 Os seawater chemistry of the proto-Pacific. Additionally we combine new 206 Pb/ 238 U zircon CA-ID-TIMS geochronology from five volcanic tuff horizons of the Yezo Group section to test and facilitate inter-basinal integration with the WIS using radio-isotopically constrained age-depth models for both sections, and quantitatively constrain the absolute timing and duration of events across the CTBI. The YG shows an almost identical Os i profile to that of the WIS, and very similar to that of other sites of the proto-Atlantic and European pelagic oceans (Turgeon and Creaser, 2008;Du Vivier et al., 2014). The characteristics of the Os i profile are radiogenic and heterogeneous (∼0.55-0.85) prior to the OAE 2, and synchronous with the inferred OAE 2 onset the Os i abruptly become unradiogenic and remain relatively homogeneous (∼0.20-0.30) before showing a gradual return to more radiogenic Os i (∼ 0.70) throughout the middle to late OAE 2. A 206 Pb/ 238 U zircon age of an interbedded tuff (HK017) in the adjacent horizon to the first unradiogenic Os i value constrains the age of the Os i inflection at 94.44 ± 0.14 Ma. This age, including uncertainty, agrees with the interpolated age of the same point in the Os i profile (94.28 ± 0.25 Ma) in the only other dated OAE 2 section, the WIS; indicating a coeval shift in seawater chemistry associated with volcanism at the OAE 2 onset at the levels of temporal resolution (ca. 0.1 Myr). Further, prior to the onset of OAE 2 an enhanced radiogenic inflection in the Os i profile of the YG is correlative, within uncertainty, with a similar trend in the WIS based on the U-Pb agedepth model. The interpolated ages, 94.78 ± 0.12 Ma and 94.66 ± 0.25 Ma for this Os i inflection in the YG and WIS, respectively, indicate that palaeocirculation was sufficient to simultaneously influence transbasinal seawater chemistry. In contrast, the pre-OAE 2 Os i profile for the GVS is disparate to that of the YG and those of the proto-Atlantic and European pelagic shelf locations. We interpret the pre OAE 2 heterogeneous Os i values (0.30-0.95) to record a palaeobasin that was regionally influenced interchangeably by both unradiogenic (hydrothermal flux) and radiogenic (continental flux) Os. We conclude that the Os i profiles from the proto-Pacific sections record both trends that are consistent globally (OAE 2 onset, syn and post OAE 2), but also show regional differences (pre OAE 2) between OAE 2 sections worldwide. As such the Os i profiles coupled with U-Pb geochronology facilitate the correlation of OAE 2 stratigraphy, and demonstrate both regional and global ocean dynamics.

Carbon sequestration in an expanded lake system during the Toarcian oceanic anoxic event

“…The observed early Toarcian perturbation to the exogenic 38 carbon cycle has been linked to volcanism of the Karoo-Ferrar large igneous province 39 (LIP) and associated release of volcanogenic CO 2 , thermogenic methane (CH 4 ) from 40 sill intrusion into Gondwanan coals, and biogenic methane from dissociation of sub-41 seafloor clathrates 3,6,[11][12][13] . Early Toarcian elevated atmospheric pCO 2 likely induced 42 climatic and environmental change 5,12,[14][15][16] by accelerating the global hydrological 43 cycle and increasing silicate weathering, thereby increasing delivery of riverine 44 nutrients to the oceans and potentially also to large inland lakes 17 . In the marine 45 realm, the consequential increase in primary productivity and carbon flux to the sea 46 floor is credited with enhancing the burial of planktonic material in relatively deep 47 continental-margin sites, whereas in shallower water semi-restricted marine basins, 48 chemical and physical water-column stratification likely aided the burial of organic 49 matter 17 .…”

“…The increased occurrence of Classopollis in tetrads 157 (relative to single specimens; Fig. 2) observed during the negative CIE interval 158 suggests stressed environmental conditions on land during the T-OAE, likely in 159 response to enhanced atmospheric pCO 2 and greenhouse-gas-induced climatic 160 warming 12,14,18,32,33 . 161…”

“…Particularly in northern Europe, the evidence points to regional to global 50 development of anoxic/euxinic (sulphide-rich) bottom waters that strongly affected 51 palaeoceanographic conditions and marine ecosystems 15,17,18 . Globally significant 52 burial of 13 C-depleted photosynthetically derived organic matter commonly produced 53 an overarching positive carbon-isotope excursion (CIE) interrupted by the 54 Previous study on a section from Bornholm, Denmark suggested a sharp 155 increase of atmospheric pCO 2 reconstructed from terrestrial leaf stomatal density at 156 the onset of the negative CIE 12 . The increased occurrence of Classopollis in tetrads 157 (relative to single specimens; Fig.…”

“…Gondwanan coals or the dissociation of sub-sea-floor gas hydrates, also resulting in 205 enhanced early Toarcian atmospheric pCO 2 levels 3,11,12,15 . The typical early Toarcian 206 δ 13 C pattern, with a stepped negative shift interrupting an overarching positive 207 excursion, has been observed in marine and terrestrial organic matter and shallow-208 water platform and deep-water pelagic carbonates and manifestly affected the entire 209 ocean-atmosphere system 3,5,6,8 .…”

“…The T-OAE is marked by 34 major changes in global geochemical cycles, with an apparently rapid negative shift 35 of as much as ~7‰ in bulk marine and terrestrial organic-carbon isotope records and 36 a typically smaller (3-6‰) negative excursion in carbonate archives and specific 37 organic compounds [3][4][5][6][7][8][9][10] . The observed early Toarcian perturbation to the exogenic 38 carbon cycle has been linked to volcanism of the Karoo-Ferrar large igneous province 39 (LIP) and associated release of volcanogenic CO 2 , thermogenic methane (CH 4 ) from 40 sill intrusion into Gondwanan coals, and biogenic methane from dissociation of sub-41 seafloor clathrates 3,6,[11][12][13] . Early Toarcian elevated atmospheric pCO 2 likely induced 42 climatic and environmental change 5,12,[14][15][16] by accelerating the global hydrological 43 cycle and increasing silicate weathering, thereby increasing delivery of riverine 44 nutrients to the oceans and potentially also to large inland lakes 17 .…”