Cryosphere carbon dynamics control early Toarcian global warming and sea level evolution
Cryosphere carbon dynamics control early Toarcian global warming and sea level evolution
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DOI：
10.1016/j.gloplacha.2018.11.003
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发表时间：
2019-01
影响因子：
3.9
通讯作者：
W. Ruebsam;B. Mayer;L. Schwark
中科院分区：
文献类型：
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作者：
W. Ruebsam;B. Mayer;L. Schwark
地球的冰冻圈是一个巨大的气候敏感的碳库，能够在全球温度上升时从永久冻土或被永久冻土和冰盖覆盖的气体库中释放二氧化碳（CO2）和甲烷（CH 4）。从这些水库释放的碳有可能进一步加速全球变暖。当今冰冻圈消亡是科学研究的一个焦点。冰冻圈碳库在中生代气候扰动中的潜在作用更是鲜为人知，目前研究不足。与之前认为侏罗纪早期持续温暖、几乎没有冰冻圈的观点相反，最近的研究已经确定了这段时间的冰库条件。在这些冰库条件下，全球变暖发生在早期Toarcian（~183 Ma），并伴随着一个主要的碳循环异常，表现在反复出现的负碳同位素偏移（CIE）。我们认为，最初火山驱动的大气温度的温和上升，在早期Toarcian触发了融化的地球的冰冻圈，在前面的Pliensbachian已扩大到中纬度地区，因此是非常容易受到变暖。沉积有机质和碳酸盐的碳同位素比值记录了温室气体的快速释放，主要是13 C贫化的CH 4或其氧化产物CO2。Toarcian沉积物显示出一系列轨道强迫的负CIE，其特征是从偏心率到周期性周期的频率变化，与更新世气候节律相当。这种模式的解释是一个自我维持的不稳定的不稳定的冰冻圈碳水库，开始在中纬度地区的偏心率是最有效的，然后有节奏地向极地发展的纬度，其中的偏心率占主导地位。到目前为止被低估的温度敏感的Pliensbachian冰冻圈的存在构成了早期Toarcian气候变化及其相关的海平面上升的必要先决条件。普林斯巴赫的冷却作用将水转移到陆地的冰冻圈，导致海平面严重下降。Pliensbachian-Toarcian边界和早期Toarcian的海侵脉冲伴随着全球气温的上升而发生，是大陆冰盖融化的结果。这种冰量效应和大量淡水排入海洋的现象，在冰冻圈消亡和海平面上升期间形成的碳酸盐的δ 18 O值异常低的情况下得到了很好的保存。碳和氧同位素比值，气候和海平面的变化，从而支持存在的早侏罗世冰冻圈，从而突出的作用，冰川海平面升降机制的主要驱动力晚Pliensbachian早期Toarcian地球动力学。
The Earth's cryosphere represents a huge climate-sensitive carbon reservoir capable of releasing carbon dioxide (CO2) and methane (CH4) from permafrost soils or gas reservoirs capped by permafrost and ice caps upon rising global temperatures. Carbon release from these reservoirs has the potential to further accelerate global warming. Present day cryosphere demise is a focus of scientific research. The potential role of cryosphere carbon reservoirs in Mesozoic climate perturbations is even lesser known and currently underinvestigated. In contrast to previous views of a constantly warm Early Jurassic period, virtually lacking a cryosphere, recent studies have identified icehouse conditions for this time interval. Following these icehouse conditions, global warming occurred during the early Toarcian (~183 Ma) and was accompanied by a major carbon cycle anomaly as manifested in recurring negative carbon isotope excursions (CIEs). We propose that an initially volcanic-driven gentle rise of atmospheric temperature in the Early Toarcian triggered a melt-down of Earth's cryosphere which during the preceding Pliensbachian had expanded to the mid-latitudes and thus was highly vulnerable to warming. The rapid release of greenhouse gases, mainly as13C-depleted CH4, or its oxidation product CO2, is recorded in the carbon isotope ratios of sedimentary organic matter and carbonates. Toarcian sediments display a series of orbitally-forced negative CIEs characterized by a frequency shift from eccentricity to obliquity cycles comparable to Pleistocene climate rhythms. This pattern is explained by a self-sustaining destabilization of labile cryosphere carbon reservoirs which started at mid-latitudes where eccentricity is most effective and then rhythmically progressed poleward to latitudes where obliquity dominates. The hitherto underestimated presence of a temperature-sensitive Pliensbachian cryosphere constituted an essential precondition for the early Toarcian climate change and its associated sea-level rise. The Pliensbachian cooling had transferred water into the terrestrial cryosphere causing a severe sea-level fall. Transgressive pulses at the Pliensbachian-Toarcian boundary and in the early Toarcian occurred concomitant to rising global temperatures and resulted from the meltdown of continental ice caps. This ice-volume effect and the massive discharge of freshwater into the oceans is well preserved in the exceptionally low δ18O values of carbonates formed during the cryosphere demise and sea-level increase. Carbon and oxygen isotope ratios, climate and sea-level shifts thus underpin the presence of an Early Jurassic cryosphere and thereby highlight the role of glacio-eustatic mechanisms as main drivers of late Pliensbachian to early Toarcian geodynamics.