化石場址顯示侏儸紀早期海洋缺氧帶來的影響

原文網址：www.sciencedaily.com/releases/2017/07/170715155134.htm

化石場址顯示侏儸紀早期海洋缺氧帶來的影響

結合化石紀錄和化學訊號，科學家得以找出全球海洋缺氧時期如何帶給侏儸紀早期的海洋生態系壓力，使其生物群集的組成變成僅有少數生物。

這項研究由德州大學奧斯汀分校地質科學院的助理教授Rowan Martindale領導，成果刊登於7月15日的《古地理學、古氣候學、古生態學》(Palaeogeography, Palaeoclimatology, Palaeoeconology)印刷版中。共同作者為德國柏林自然史博物館演化和生物多樣性科學研究所的主任Martin Aberhan。

研究焦點放在加拿大最近發現的化石場址，其位於亞伯達省西南方班夫國家公園附近的Ya Ha Tinda牧場。在1億8300萬年前的侏儸紀早期此地曾被淺海覆蓋，形成的化石場址記錄了當時生活於此的生物。

這處化石場址為托阿爾階海洋缺氧事件(Toarcian Oceanic Anoxic Event)的科學紀錄加上一筆，該事件發生期間淺層海水處於低氧狀態，理論認為是由大型火山噴發導致。在先前由維吉尼亞理工的Benjamin Gill和Theodore Them主持的研究計畫中，他們從岩石蒐集的地球化學數據證實了此處有受到海洋缺氧事件的影響。侏儸紀早期岩石所處環境的氧濃度會影響岩石保存的碳含量與類型，使得地球化學紀錄成為追蹤缺氧事件的重要方法。

德州大學奧斯汀分校地質科學系的研究人員Martindale表示：「我們擁有這些絕佳的地球化學紀錄顯示海洋缺氧事件的時間歷程。基於此框架我們可以進一步觀察底棲族群，也就是生活在海底的生物而提出這項問題：『此處的生物群集對缺氧事件產生什麼樣的反應？』」

化石紀錄顯示在缺氧事件發生之前，Ya Ha Tinda的海洋生物群集相當多樣化，成員包括魚類、魚龍(已滅絕，狀似海豚的海洋爬蟲類)、海百合、龍蝦、蛤蜊、牡蠣、菊石和箭石(像魷魚的八腕動物)。在缺氧事件期間生物群集遭到毀滅，雖然之後有重新建立起來，但其中的生物種類已經大幅減少。像是在缺氧事件之前群集中最為豐富的蛤蜊種類就被完全消滅而由別種蛤蜊取代。

在事件中生存下來以及往後出現的蛤蜊都比事件之前小的多，代表低氧濃度限制了蛤蜊的體型。

Ya Ha Tinda紀錄中事件期間與之前的海洋生物變化跟歐洲化石場址紀錄有相似之處。未參與此研究，英國里茲大學的古生物學資深講師Crispin Little表示這些化石場址之間的相似之處強烈顯示缺氧事件的影響範圍相當廣大。

Little表示：「這證實了前人研究中，T-OAE(缺氧事件)確實影響到全世界的說法。」

然而，當其他地方正從缺氧事件復甦時，Ya Ha Tinda的環境仍持續面臨著相當壓力。即使是那些小型而強韌的雙殼貝，要在這裡存活都是一件相當困難的事。

Martindale表示：「關於(Ya Ha Tinda)的復甦過程，有個很有趣的地方是在我們認為應該能看到群集復甦的時候，實際上看到的生物數目卻很少。」

Martindale說化石顯示環境受到了某些區域性壓力使得氧濃度仍然低落。還需要更多研究才能解釋為什麼Ya Ha Tinda的生物回復速率跟其他地區並非一致。

共同作者Martin Aberhan表示，既然海洋缺氧是氣候變遷的副作用之一，回顧過往的海洋生物群集可以讓我們窺見現在和未來的氣候變遷可能造成的影響。

Aberhan表示：「從此研究中我們能學到的一課是，由人類的時間尺度來看，氣候導致的(生態)壓力可以影響很長一段時間，在長達數十萬年中都不會有復甦跡象。另外，氣候危機發生前後的生物群集不管是組成與生態功能，可能都會有十分大的差異。」

Fossil site shows impact of early Jurassic's low oxygen oceans

Using a combination of fossils and chemical markers, scientists have tracked how a period of globally low ocean-oxygen turned an Early Jurassic marine ecosystem into a stressed community inhabited by only a few species.

The research was led by Rowan Martindale, an assistant professor at The University of Texas at Austin Jackson School of Geosciences, and published in print in Palaeogeography, Palaeoclimatology, Palaeoeconology on July 15. The study was co-authored by Martin Aberhan, a curator at the Institute for Evolution and Biodiversity Science at the Natural History Museum in Berlin, Germany.

The study zeroes in on a recently discovered fossil site in Canada located at Ya Ha Tinda Ranch near Banff National Park in southwest Alberta. The site records fossils of organisms that lived about 183 million years ago during the Early Jurassic in a shallow sea that once covered the region.

The fossil site broadens the scientific record of the Toarcian Oceanic Anoxic Event, a period of low oxygen in shallow ocean waters which is hypothesized to be triggered by massive volcanic eruptions. The Oceanic Anoxic Event was identified at this site by the geochemical record preserved in the rocks. These geochemical data were collected in a previous research project led by Benjamin Gill and Theodore Them of Virginia Tech. The oxygen level of the surrounding environment during the Early Jurassic influences the type and amount of carbon preserved in rocks, making the geochemical record an important method for tracking an anoxic event.

"We have this beautiful geochemical record that gives us a backbone for the timing of the Oceanic Anoxic Event," said Martindale, a researcher in the Jackson School's Department of Geological Sciences. "So with that framework we can look at the benthic community, the organisms that are living on the bottom of the ocean, and ask 'how did this community respond to the anoxic event?"

The fossils show that before the anoxic event, the Ya Ha Tinda marine community was diverse, and included fish, ichthyosaurs (extinct marine reptiles that looked like dolphins), sea lilies, lobsters, clams and oysters, ammonites, and coleoids (squid-like octopods). During the anoxic event the community collapsed, restructured, and the organisms living in it shrunk. The clams that were most abundant in the community before the anoxic event were completely wiped out and replaced by different species.

The clams that survived during and after the event were much smaller than the clams from before the event, suggesting that low oxygen levels limited their growth.

The sea life recorded at Ya Ha Tinda before and during the anoxic event is similar to fossils found at European sites. Crispin Little, a senior lecturer in paleontology at The University of Leeds who was not involved with the research, said that the similarity between the sites underscores the widespread nature of the anoxic event.

"This confirms previous work suggesting that the T-OAE (anoxic event) was genuinely a global event," Little said.

However, while other sites were recovering from the anoxic event, the environment at Ya Ha Tinda continued to face stress. Even for small, hardy bivalves, life was tough.

"One of the interesting things about the recovery [at Ya Ha Tinda] is that we actually see fewer individuals at a time when we're supposed to be seeing community recovery," Martindale said.

The fossils suggest that the environment was undergoing local stresses that kept oxygen low, Martindale said. More research is needed to untangle why life at Ya Ha Tinda didn't recover at the same rate as other places.

Since the oceanic anoxic event was a side-effect of climate change, looking back at ancient marine communities could be a window into the potential impacts of ongoing and future climate change, said co-author Martin Aberhan.

"One lesson we can learn from this study is that, on a human time scale, climate-related stresses can have very long-lasting effects, with no signs of recovery for hundred thousands of years, and that the communities before and after a climatic crises can look quite different in composition and ecological functioning," Aberhan said.

原始論文：Rowan C. Martindale, Martin Aberhan. Response of macrobenthic communities to the Toarcian Oceanic Anoxic Event in northeastern Panthalassa (Ya Ha Tinda, Alberta, Canada). Palaeogeography, Palaeoclimatology, Palaeoecology, 2017; 478: 103 DOI: 10.1016/j.palaeo.2017.01.009

引用自：University of Texas at Austin. "Fossil site shows impact of early Jurassic's low oxygen oceans." ScienceDaily. ScienceDaily, 15 July 2017.