Feedbacks upon feedbacks: Rock weathering and the climate

反馈之上的反馈：岩石风化与气候

Since the early 1980s, Earth scientists have understood that erosion and weathering of rock slowly removes CO2 from the atmosphere, regulating Earth’s climate on geological timescales. But recent studies have shown that erosion can also emit CO2 by oxidizing organic carbon contained in eroding sediments. It hasn’t been clear how this competition between removal by rock weathering and emission by organic carbon weathering ends up affecting Earth’s climate. 自20世纪80年代初以来，地球科学家们就已经认识到，岩石的侵蚀和风化会缓慢地从大气中移除二氧化碳，从而在地质时间尺度上调节地球气候。但最近的研究表明，侵蚀过程也会通过氧化沉积物中含有的有机碳来排放二氧化碳。目前尚不清楚岩石风化带来的碳移除与有机碳风化带来的碳排放之间的这种竞争，最终会如何影响地球气候。

A new study in the journal Nature Communications uses the geological past to test how these competing effects added up. Doctor Madeleine Stow of the University of Oxford, with colleagues from across the UK and France, examined a volcanically triggered episode of global warming that happened in the early part of the Jurassic period, 183 million years ago, known as the “Toarcian Ocean Anoxic Event.” They found that eroding organic carbon amplified climate warming at the time, suggesting that the same process may apply to modern climate change. But the extent to which the past is prologue is uncertain. 发表在《自然-通讯》（Nature Communications）杂志上的一项新研究利用地质历史来验证这些相互竞争的影响是如何叠加的。牛津大学的Madeleine Stow博士与来自英国和法国的同事们一起，研究了1.83亿年前侏罗纪早期发生的一起由火山活动引发的全球变暖事件，即“托阿尔期海洋缺氧事件”（Toarcian Ocean Anoxic Event）。他们发现，当时有机碳的侵蚀加剧了气候变暖，这表明同样的过程可能也适用于现代气候变化。但历史在多大程度上是未来的序章，目前尚不确定。

Sampling the air using sediments from the seabed

利用海底沉积物“采样”大气

The Toarcian warming event is one of a dozen or so periods of climate change in the geological past that were triggered by enormous volcanic phenomena known as large igneous provinces. Several are associated with mass extinctions, including the Great Dying at the end of the Permian period, which was caused by the Siberian Traps Large Igneous Province. The Toarcian event was triggered by massive volcanic eruptions across South Africa and Antarctica, which were joined together at the time. The resulting 6° to 7° C of global warming shuffled the makeup of plant and dinosaur species on land and caused a mass extinction of corals and other marine species. 托阿尔期变暖事件是地质历史上十几次由被称为“大火成岩省”（Large Igneous Provinces）的巨大火山现象引发的气候变化时期之一。其中几次事件与大规模灭绝有关，包括由西伯利亚地盾大火成岩省引起的二叠纪末期“大灭绝”。托阿尔期事件则是由当时连接在一起的南非和南极洲地区的大规模火山喷发所引发的。由此导致的全球气温升高6°至7°C，改变了陆地上植物和恐龙物种的构成，并导致了珊瑚及其他海洋物种的大规模灭绝。

“This event had been well studied before. We understand its drivers, we understand how it caused mass extinctions, and it’s driven by this Large Igneous Province release,” explained University of Oxford professor Bob Hilton, a coauthor and principal investigator in the study. “这一事件此前已被深入研究。我们了解其驱动因素，了解它如何导致大规模灭绝，并且它是由大火成岩省的物质释放所驱动的，”该研究的合著者兼首席研究员、牛津大学教授Bob Hilton解释道。

Organic carbon in rocks ranges from visible debris from fossil leaves and wood to molecular remains of plankton, algae, and microbes. In past global warming events, like the Toarcian, so much organic matter was buried at sea that the resulting shales are black with organic carbon. Later, after plate tectonics raises such sediments to the land surface, they can be eroded, and the organic carbon within them can be weathered into CO2. 岩石中的有机碳种类繁多，从化石叶片和木材的可见碎片，到浮游生物、藻类和微生物的分子残留物，应有尽有。在像托阿尔期这样的过去全球变暖事件中，大量的有机物质被埋藏在海底，形成的页岩因富含有机碳而呈现黑色。后来，当地壳构造运动将这些沉积物抬升到陆地表面时，它们会受到侵蚀，其中的有机碳便会风化成二氧化碳。

To measure how much organic carbon was weathered on land during the Toarcian, Stow and colleagues turned to isotopes of the element rhenium extracted from rocks deposited on the seabed at the time. Rhenium works as a tracer of organic carbon oxidation because it binds chemically with organic matter in seabed sediments. When organic carbon is weathered on eroding land, it’s released to the atmosphere as CO2 gas. But the rhenium that was bound to the organic carbon gets washed through rivers into the ocean, where it is incorporated into new seabed sediments. There, it acts as a tracer of the organic carbon that was oxidized from the older sediments. 为了测量托阿尔期陆地上风化了多少有机碳，Stow及其同事转向了从当时沉积在海底的岩石中提取的铼（rhenium）同位素。铼可以作为有机碳氧化的示踪剂，因为它能与海底沉积物中的有机物质发生化学结合。当有机碳在受侵蚀的陆地上风化时，它会以二氧化碳气体的形式释放到大气中。但与有机碳结合的铼会被河流冲入海洋，并被纳入新的海底沉积物中。在那里，它充当了从旧沉积物中氧化出来的有机碳的示踪剂。

The intensity of organic carbon oxidation changes the ratio of the isotope rhenium 187 to rhenium 185. This makes the ratio of these two isotopes in sediments a measure of the organic carbon weathering at the time. The team used a 1,300-meter rock core of sediments deposited from the late Triassic to the early Jurassic. It was drilled in the 1960s in Wales and is now stored by the British Geological Survey. The concentrations of rhenium in the rock are as low as one billionth of a gram per gram of rock, requiring exquisitely sensitive techniques to measure it. Recent improvements in the sensitivity of mass spectrometers have been “a bit of a game changer” in allowing studies like this, Hilton said. 有机碳氧化的强度会改变铼-187与铼-185的同位素比值。这使得沉积物中这两种同位素的比值成为衡量当时有机碳风化程度的指标。研究团队使用了一根从三叠纪晚期到侏罗纪早期沉积的1300米长岩芯。该岩芯于20世纪60年代在威尔士钻探，目前由英国地质调查局保存。岩石中铼的浓度低至每克岩石十亿分之一克，需要极其灵敏的技术才能测量。Hilton表示，近期质谱仪灵敏度的提升在促成此类研究方面起到了“改变游戏规则”的作用。

The team chipped shale samples from different points along the drill core, with each sampling location representing a different point in time during the warming event. They digested the rock samples in a succession of different acids to break down the minerals and the organic matter, and after several further preparation steps, the rhenium isotopes were measured using “Inductively Coupled Plasma Mass Spectrometry” (ICP-MS). The rhenium isotope values they measured changed as the Toarcian warming event unfolded, indicating that organic carbon weathering intensified as the climate heated up. 研究团队从岩芯的不同点位凿取了页岩样本，每个采样点代表了变暖事件期间的不同时间点。他们用一系列不同的酸对岩石样本进行消解，以分解其中的矿物质和有机物。经过后续的几个制备步骤，研究人员使用“电感耦合等离子体质谱仪”（ICP-MS）测量了铼同位素。他们测得的铼同位素数值随着托阿尔期变暖事件的演变而发生变化，这表明随着气候变暖，有机碳的风化作用也在加剧。

“This is a really important paper because it is one of the first to use rhenium isotopes in a past geologic context, and that really opens up this system to be able to study further in the past and to study others of these anomalous events to gain more understanding of our current Earth system,” said Katherine Grant of the Lawrence Livermore National Laboratory, who was not involved in the study. Professor Jeremy Caves Rugenstein of Colorado State University, who was also not involved in the study, highlighted the importance of the paper in the context of earlier work by Hilton’s group. “This group… has revolutionized our understanding of how geologic organic carbon interacts with climate,” he said. “这是一篇非常重要的论文，因为它是首批在地质历史背景下使用铼同位素的研究之一。这确实为进一步研究更久远的过去以及其他类似的异常事件打开了大门，从而让我们对当前的地球系统有更深入的了解，”未参与此项研究的劳伦斯利弗莫尔国家实验室的Katherine Grant表示。同样未参与研究的科罗拉多州立大学教授Jeremy Caves Rugenstein强调了该论文在Hilton团队早期工作背景下的重要性。他说：“这个团队……彻底改变了我们对地质有机碳如何与气候相互作用的理解。”

Mind the gap

注意差距

“Our modern measurements of this process suggest if you warm up the planet, you should weather this material more intensely,” said Hilton, “and that’s exactly what we see in the core as we had this warming event. Weathering breakdown of rocks can actually be acting as a carbon dioxide source, and this can be quite large.” “我们对这一过程的现代测量表明，如果你让地球变暖，这些物质的风化程度应该会更剧烈，”Hilton说，“这正是我们在岩芯中观察到的变暖事件期间的情况。岩石的风化分解实际上可以作为二氧化碳的来源，而且其规模可能相当大。”

A 2024 study by Professor Isabel Fendley of Penn State and colleagues used the same rock core to assess the amount of volcanic CO2 emitted in the Toarcian warming event. Using mercury as a tracer for the volcanic eruptions, they concluded that volcanic CO2 alone was insufficient to drive the warming, so there must have been an additional source of greenhouse gas. 宾夕法尼亚州立大学的Isabel Fendley教授及其同事在2024年进行的一项研究使用了同一根岩芯，评估了托阿尔期变暖事件中排放的火山二氧化碳总量。他们利用汞作为火山喷发的示踪剂，得出结论认为，仅靠火山二氧化碳不足以驱动当时的变暖，因此必然存在额外的温室气体来源。