Astronomical pacing of the global silica cycle recorded in Mesozoic bedded cherts

The global silica cycle is an important component of the long-term climate system, yet its controlling factors are largely uncertain due to poorly constrained proxy records. Here we present a ∼70 Myr-long record of early Mesozoic biogenic silica (BSi) flux from radiolarian chert in Japan. Average low-mid-latitude BSi burial flux in the superocean Panthalassa is ∼90% of that of the modern global ocean and relative amplitude varied by ∼20–50% over the 100 kyr to 30 Myr orbital cycles during the early Mesozoic. We hypothesize that BSi in chert was a major sink for oceanic dissolved silica (DSi), with fluctuations proportional to DSi input from chemical weathering on timescales longer than the residence time of DSi (<∼100 Kyr). Chemical weathering rates estimated by the GEOCARBSULFvolc model support these hypotheses, excluding the volcanism-driven oceanic anoxic events of the Early-Middle Triassic and Toarcian that exceed model limits. We propose that the Mega monsoon of the supercontinent Pangea nonlinearly amplified the orbitally paced chemical weathering that drove BSi burial during the early Mesozoic greenhouse world.


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samples analysed, these elements will not be discussed in this study.

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To estimate the BSi content from the major element contents of bedded chert, it is 26 necessary to know SiO 2 and Al 2 O 3 contents of the terrigenous material in bedded chert.

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Previous studies on bedded chert assumed that SiO 2 and Al 2 O 3 contents of terrigenous material 28 as 52.8% and 26.1%, respectively, with SiO 2 /Al 2 O 3 ratio of 2.02, which based on the chemical 29 composition of illite 3, 4, 5 ( Fig. 2A). However, the terrigenous material accumulated in the 30 modern pelagic ocean contains not only illite, but also quartz and feldspars with their contents 31 being 20 to 40% and 15 to 30%, respectively, based on the mineral compositional analysis of 32 3 pelagic red clay 6,7 . Therefore, SiO 2 /Al 2 O 3 ratio of terrigenous material accumulated in the 33 modern pelagic ocean is 3 to 4.5 based on the major element chemical analysis of pelagic red 34 clay 6, 7 ( Fig. 2A). Thus, the terrigenous material in bedded chert is probably more SiO 2 rich, and 35 its SiO 2 /Al 2 O 3 ratio should be larger than previously estimated.

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Assuming the smallest SiO 2 content among all the analyzed shale samples should be close 37 to the SiO 2 content of terrigenous material in bedded chert, the calculated BSi contents in 38 individual chert and shale beds range from 75 to 93% and 0 to 75% with average values of 81% 39 and 20%, respectively. The standard deviations of calculated BSi contents of the individual 40 chert and shale beds are 7.8% and 14% with a relative standard deviation of 9.6% and 70%, 41 respectively ( Fig. 2A). The accumulation amounts of BSi and terrigenous material per one 42 chert-shale couplet per unit area range from 2.5 to 14 g cm -2 and from 0.5 to 5.1 g cm -2 with 43 average values of 6.8 g cm -2 and 3.0 g cm -2 , respectively (

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Kamuikotan Terrane, Hokkaido, northern Japan 12, 13 ( Fig. 1 and Supplementary Fig. 1). The 57 paleolatitude of the Pisenaizawa section can be inferred as low latitude on the basis of the 58 nearby Upper Cretaceous fore-arc basin sequence in the low latitude 14 (16.7• +11.0/-9.8•N) and 59 plate motion direction of the Izanagi Plate 15 . The distance that these terranes traveled prior to 60 accretion is not known with certainty, but a rough estimate of 2100-4200, 2400-4800, 3000-61 6000 km can be made on the basis of approximately ~70-Myr (Early Triassic-Early Jurassic), 62 ~80-Myr (Early Triassic-Middle Jurassic), and ~100-Myr (Late Triassic-Early Cretaceous) 63 travel history, respectively, and a rough convergence rate of 3-6 cm yr −1 15 . The Lower Triassic 64 5 bedded chert deposited at the southern middle latitude (34°S ± 8°) of western Panthalassa was 65 found in Waiheke island of the Waipapa composite Terrane, New Zealand 16 (Fig. 1). The Lower

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North America (Fig. 1). Thus, it can be concluded that bedded chert was widely deposited at 69 least in the low to middle latitude in the both hemispheres of the eastern and western 70 Panthalassa during the Early Triassic to Early Jurassic (Fig. 1).

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The superocean Panthalassa comprised an area of 80-90% in area of the global ocean 72 during the Early Triassic to Early Jurassic ( Fig. 1) 19 . Although its distribution could have been 73 extended to higher latitudes, there are no high latitude pelagic records available at this moment.

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Assuming that bedded chert covered the area of the low latitude Panthalassa between 30°±10°

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We also compiled the BSi burial fluxes for bedded chert sequences of the area other than 79 the Inuyama area to support the claim that the BSi records of the Inuyama bedded chert is

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Although the calculated relative amplitudes of orbital-scale chemical weathering rates by 123 the simple weathering model is negligible, chemical weathering is a highly non-linear system.