A satellite image of the Northern Gulf of Mexico, where 'dead zones' can reach the size of New Jersey. Photo Credit: NASA
Oxygen-starved ocean “dead zones,” where fish and animals cannot survive, 几十年来，由于人类的农业和工业活动，在开阔的海洋和沿海水域不断扩大吗. Trying to predict the scale and location of future dead zones, scientists have looked to the past for historical clues.
Today’s largest open ocean dead zone, located in the eastern Pacific Ocean, 八百万年前由于海洋中营养成分的增加而出现, an international team of scientists reported in the Proceedings of the National Academy of Sciences.
While the sources of nutrient enrichment today may be different, 造成科学家所谓的“缺氧区”的机制保持不变, 电子游戏软件地球与环境科学助理教授Xingchen“Tony”Wang说, a lead author of the report. 更好地了解过去的海洋死亡区，可能有助于未来的海洋保护工作.
Xingchen “Tony” Wang (Lee Pellegrini)
“In order to better protect marine ecosystems and manage fisheries, 预测海洋“死亡区”未来将如何演变是至关重要的,” Wang said.
沿海海洋死区主要是由人类在陆地上使用的过量营养物质流动造成的, such as fertilizer application. In the Northern Gulf of Mexico, 每年由密西西比河输送的人为营养物造成了一个和新泽西州一样大的死区.
These zones also occur naturally in the open ocean, with the largest found in the eastern Pacific Ocean. “目前还不清楚随着地球变暖，这些死亡区将如何变化. So, 我们电子游戏正规平台了东太平洋死亡区的历史，以便更好地预测它的未来行为,” Wang said.
The researchers from universities in the U.S., Canada, Taiwan, Germany, 在人类活动开始影响海洋之前，澳大利亚就开始确定开放海洋死亡区的演变, Wang said. In addition, did these dead zones always exist? If so, why?
电子游戏正规平台小组在今天最大的海洋死亡区附近检查了海洋沉积物的化学成分, located in the eastern Pacific Ocean. 该团队获得了1200万年前的沉积物样本——海洋活动的“历史书”，并分析了微化石中含有的氮, known as foraminifera.
“海洋沉积物中的有孔虫化石主要由碳酸钙构成，几十年来它们一直被用来电子游戏正规平台过去的气候变化,” said Tianshu Kong, 她是王的实验室里电子游戏正规平台有孔虫的博士生. “在海洋沉积物中发现的大多数有孔虫实际上来自地表水, so they can tell us what happened in the upper ocean.”
The team looked in the dead zones for signs of denitrification, 当氧气含量很低时，微生物会转而使用硝酸盐来为它们的生物活动提供动力. Nitrogen has two stable isotopes, nitrogen 14 and nitrogen-15, 在反硝化过程中，微生物更倾向于消耗较轻的氮-14同位素.
When oxygen deficient zones expand, denitrification zones also expand, raising the nitrogen-15 to nitrogen-14 ratio of the remaining nitrate, 然后通过海洋生态系统中氮的循环记录在有孔虫等海洋生物中, according to the report.
“通过分析海洋沉积物中有孔虫的氮-15和氮-14的比例, we can reconstruct the history of the extent of oxygen deficient zones,” said Wang, whose work on the project was funded in part by the Simons Foundation.
In addition, 电子游戏正规平台人员分析了同一沉积物的磷和铁含量, which reveal the ancient nutrient content in the deep Pacific Ocean, according to the report.
“Deep-ocean nutrient content is hard to reconstruct and our record is the first of its kind over the past 12 million years; its trends have important implications for the global carbon cycle and climate change,” said Woodward W. 他是这项电子游戏正规平台的合著者之一，也是加州理工学院的教授.
Today’s largest “dead zones” are located in the eastern Pacific Ocean. 符号和颜色表示缺氧水的位置和厚度, with darker color indicating thicker 'dead zones.' Photo Credit: Tianshu Kong.
沉积记录表明，在过去的800万年里，最大的开放海洋死亡区逐渐扩大, said Wang, 他和马克斯·普朗克化学电子游戏正规平台所的同事一起参与了这个项目, Princeton University, National Taiwan University, University of Toronto, Texas A&M University, University of Western Australia, and Caltech.
“此外，这些死亡区的扩大主要是由营养物质富集引起的，”他说. “这种机制类似于今天沿海水域死区的形成, except that humans are responsible for the current nutrient enrichment.”
这些发现可能有助于更好地预测开放海洋死亡区的未来行为, according to the report. 例如，人类活动一直在向海洋中添加越来越多的氮. 它们可以支持改进气候和海洋模型的需求，以更好地衡量人为氮对开放海洋脱氧过程的影响.
数据显示，在800万年前，海洋中的营养成分含量要低得多，这让电子游戏正规平台人员感到震惊.
“自800万年前以来，营养物质的增加很可能是由于陆地风化和侵蚀的加剧，这将增加向海洋输送磷的量,” said Fischer.
“In addition, 陆地生态系统在800万到600万年前经历了一次重大转变,” Wang said. “许多森林被密度较低的草地所取代，这被称为C4生态系统的扩张. With more grassland, soil erosion might have increased during this period, 它会引发更多的有机营养物质向海洋转移.”
王说，这项电子游戏正规平台的下一步可能是确定人类活动流入海洋的氮是如何影响海洋的营养循环的.
“The key questions lie in our coastal zones, where the most anthropogenic nitrogen enters the ocean,” Wang said. “如果大部分人为氮在沿海地区被清除——主要是通过沉积物中的反硝化作用——那么这可能会减轻对整个海洋的影响. 我们在不列颠哥伦比亚省的电子游戏正规平台小组目前正在墨西哥湾北部做一些工作，以更好地了解海洋中人为氮的命运.”
Ed Hayward | University Communications | January 2023