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The calcareous nannofossil Prinsiosphaera achieved rock-forming abundances in the latest Triassic of western Tethys: consequences for the δ13C of bulk carbonate

機(jī)譯：在最新的特提斯西部三疊紀(jì)，鈣質(zhì)納米化石的白堊紀(jì)藻類達(dá)到了成巖的豐度：散裝碳酸鹽的δ13C的后果

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摘要

The onset of pelagic biomineralization was a milestone in the history of the long-term inorganic carbon cycle: as soon as calcareous nannofossils became major limestone producers, the pH and supersaturation state of the global ocean were stabilized (the so-called mid-Mesozoic revolution). But although it is known that calcareous nannofossils were abundant already by the end of the Triassic, no estimates exist on their contribution to hemipelagic carbonate sedimentation. With this work, we estimate the volume proportion of Prinsiosphaera, the dominant late Triassic calcareous nannofossil, in hemipelagic and pelagic carbonates of western Tethys. The investigated Upper Triassic lime mudstones are composed essentially of microspar and tests of calcareous nannofossils, plus minor bioclasts. Prinsiosphaera had become a significant component of lime mudstones since the late Norian, and was contributing up to ca. 60% of the carbonate by the late Rhaetian in periplatform environments with hemipelagic sedimentation. The increasing proportion of Prinsiosphaera in upper Rhaetian hemipelagic lime mudstones is paralleled by an increase of the δ13C of bulk carbonate. We interpreted this isotopic trend as related to the diagenesis of microspar, which incorporated respired organic carbon with a low δ13C when it formed during shallow burial. As the proportion of nannofossil tests increased, the contribution of microspar with low δ13C diminished, determining the isotopic trend. We suggest that a similar diagenetic effect may be observed in many Mesozoic limestones with a significant, but not yet dominant, proportion of calcareous plankton.
機(jī)譯：中上層生物礦化的開始是長(zhǎng)期無機(jī)碳循環(huán)歷史上的一個(gè)里程碑：鈣質(zhì)納米化石成為主要的石灰石生產(chǎn)商后，全球海洋的pH值和過飽和狀態(tài)就穩(wěn)定了（所謂的中生代革命） ）。但是，盡管眾所周知，到三疊紀(jì)末，鈣質(zhì)納米化石已經(jīng)很豐富，但沒有估計(jì)其對(duì)半碳酸鹽沉積的貢獻(xiàn)。通過這項(xiàng)工作，我們估計(jì)了特提斯西部的半碳酸鹽巖和中上層碳酸鹽巖中主要的晚三疊世鈣質(zhì)納米化石的白粉蟲的體積比例。所研究的上三疊統(tǒng)石灰質(zhì)泥巖主要由微晶石和鈣質(zhì)納米化石以及少量生物碎屑組成的試驗(yàn)組成。自晚諾里安時(shí)代以來，白頭翁藻已成為石灰泥巖的重要組成部分，并在大約Rhaetian晚期的碳酸鹽巖中約有60％處于碳酸鹽巖沉積的半平臺(tái)環(huán)境中。上萊茵半球半灰質(zhì)灰泥巖中粉虱的比例增加與塊狀碳酸鹽的δ13C升高平行。我們將這種同位素趨勢(shì)解釋為與微晶巖的成巖作用有關(guān)，微晶巖在淺埋時(shí)形成的呼吸碳有機(jī)物具有較低的δ13C。隨著納米化石測(cè)試比例的增加，低δ13C的微晶石的貢獻(xiàn)減少，從而確定了同位素趨勢(shì)。我們建議在許多中生代石灰?guī)r中觀察到類似的成巖作用，其中鈣質(zhì)浮游生物的比例很大，但尚未占主導(dǎo)地位。

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