沉积物可能支持地中海大洪水假说
Sediments May Support the Mediterranean Megaflood Hypothesis

Sediments May Support the Mediterranean Megaflood Hypothesis

沉积物可能支持地中海大洪水假说

六百万年前,地中海和大西洋被切断。非洲板块和欧亚板块的碰撞导致里夫山脉和贝提克山脉(分别位于摩洛哥北部和西班牙南部)的抬升,至少封闭了两个连接水体的大海峡。

这一事件导致了地中海盐度的突然增加,被称为Messinian盐度危机(MSC),这又导致了地中海大量石膏和盐沉积物的形成。

科学家们仍在争论关闭地中海西部水道后,地中海的海平面究竟发生了什么变化。一种可能性是水迅速蒸发,海平面下降数千米。只有一些咸水或半咸水湖泊可能幸存下来。在可能是地中海盆地古表面的侵蚀河道和沉积物似乎支持这一理论。

Zanclean洪水假说

当直布罗陀海峡在533万年前重新开放时,干旱的地中海时期可能很快就结束了,这导致了一场巨大的洪水,仅用两年时间就将这个干涸的盆地填满。地震反射成像技术是一种依靠声波来成像地下的技术,它揭示了在几千米深的第四系沉积物下,一个巨大的侵蚀通道横跨海峡。这条古老的海峡有几百米深,390公里长,从大西洋的Cdiz湾一直延伸到地中海的阿尔及利亚盆地。研究人员认为,这条水道可能是在大西洋洪水涌入地中海时开凿的。

这两个视频总结了我们基于地质和地球物理数据对500万年前地中海发生的事情的解释。首先,由于岩石圈的撕裂和下沉,大西洋和地中海之间的连接可能已经关闭。

为了寻找支持这一理论的证据(称为赞克林洪水假说),一个国际研究小组最近发现了可能是洪水沉积的地下沉积物。“一个经常出现的问题是‘洪水过后,所有的沉积物都堆积在哪里?’”’”西班牙巴塞罗纳Jaume Almera地球科学研究所的地球物理学家Daniel Garcia-Castellanos说。

加西亚-卡斯特利亚诺斯和他的团队利用计算机模拟重现了海峡的开口和随后的洪水,确定了可能堆积沉积物的低流量区域。然后,他们查看这些地区的地震剖面,看看是否能找到沉积物。

复合地震剖面显示了位于直布罗陀海峡东侧的墨西尼侵蚀面(MSC,紫色线)。这种不整合被解释为在晚墨西期盐化危机或上新世早期在中新世沉积物中开挖的侵蚀通道。来源:Garcia-Castellanos等。阿尔,抄送4

在一个古代火山锥的背风面,在洪水的预测路径中,他们发现了一个巨大的沉积物沉积。在地震剖面上,它以无定形的斑点状出现,与周围整齐的分层的海洋沉积层形成对比,表明沉积速度很快。它的形状和大小似乎也与洪水可能的方向相匹配。

加西亚-卡斯特利亚诺斯说:“这种形状与数值模型和我们在其他大洪水环境中看到的形状是一致的。”

2018年,同一组研究人员在马耳他悬崖的水下峡谷附近发现了类似的沉积物。马耳他悬崖是分隔地中海东部和西部盆地的天然屏障。这个峡谷,被称为Noto峡谷,是洪水最有可能漫过悬崖填满地中海东部盆地的地方。

这些模拟的主要局限性是缺乏有关古代海底地形的知识。加西亚-卡斯特利亚诺斯解释说:“我们使用了现在的海底形态,因为我们不知道过去是怎样的。”“然而,这些模拟给了我们一个定量的概念,即这样的气流可以将每种颗粒大小输送到多远的地方。”

研究人员在《地球科学评论》上发表了一篇关于赞克尔洪水假说的论文。

洪水还是不洪水

赞斯尔洪水假说也有批评者。许多研究地中海的学者甚至不确定地中海的实际干化程度是否远低于现代水平。

干燥理论的主要问题之一是在MSC期间沉积了大量的盐和石膏。如果海水立即蒸发,现在的地中海就会留下一层不超过30米厚的盐。相比之下,与MSC相关的盐层在一些地区厚达一公里。

赞克尔假说的批评者说,要积累这么多的盐,水必须能够从开阔的海洋流入大海,尽管是以一种受限的方式,可能会限制密度更大的咸水的流出。这些稠密的水会下沉到海洋的最深处,在那里矿物质会沉淀。

“这场辩论不会在明天结束。有争议的证据是古代河道网络被认为是古代河流系统的遗迹。目前的研究表明,这些水道可能是在海水沉入海底最深处时,由密度大的咸水冲刷而成的。类似的影响可能在直布罗陀海峡底部形成侵蚀通道。

巴塞罗那大学(University of Barcelona)的研究员哈维尔·加西亚·维加斯(Javier Garcia Veigas)没有参与这项新研究,他在寻找一种不同的方法来评估这个问题。生物活性等因素会改变海水中某些化学物质重同位素与轻同位素的比例。这些差异在不同同位素组成的水环境中形成的矿物中是可以测量的。然而,加西亚·维加斯并没有发现东地中海盆地和西地中海盆地形成的岩石之间存在明显的同位素差异,这表明它们之间仍然存在联系。

加西亚·维加斯说:“我认为社区的共识正在慢慢地偏离大洪水假说,并逐渐向地中海没有完全干涸的情况发展。”“然而,在钻探活动到达地中海深处盆地并确认我们在地震剖面上看到的确实是在Messinian期间形成的盐之前,我们还没有确凿的证据。”

加西亚-卡斯特利亚诺斯承认,并不是所有人都认同赞克利洪水假说。“这篇文章中总结的所有证据都可能有其他的解释,”他说。“所以这场辩论不会在明天结束。”

Javier Barbuzano (@javibarbuzano),自由科学记者

Six million years ago, the Mediterranean Sea was cut off from the Atlantic Ocean. The collision of the African and Eurasian tectonic plates caused the uplift of the Rif and Betic Mountains (in northern Morocco and southern Spain, respectively), closing at least two large straits or channels that connected the water bodies at the time.

This event caused a sudden increase in salinity in the Mediterranean, known as the Messinian Salinity Crisis (MSC), which in turn led to the formation of massive gypsum and salt deposits throughout the Mediterranean.

Scientists are still debating about what happened to sea levels in the Mediterranean after its western waterways closed. One possibility is that water evaporated quickly and sea levels fell by thousands of meters. Only a series of salty or brackish lakes might have survived. Eroded channels and sediment deposits in what might have been the ancient surface of the exposed Mediterranean basin seem to support this theory.

Zanclean Flood Hypothesis

The period of the dry Mediterranean might have ended quickly when the Strait of Gibraltar reopened 5.33 million years ago, causing a massive flood that refilled the empty basin in just 2 years. Seismic reflection imaging, a technique that relies on sound waves to image what lies underground, has revealed that beneath kilometer-deep Quaternary sediments, a huge erosion channel extends across the strait. The ancient channel, several hundred meters deep and 390 kilometers long, stretches from the Gulf of Cdiz (in the Atlantic) to the Algerian Basin (in the Mediterranean). Researchers think that this channel might have been carved during the flood as Atlantic waters rushed in to refill the Mediterranean.

These two videos summarize our interpretation of what happened to the Mediterranean ~5 million years ago based on geological & geophysical data. First, the connections between the Atlantic and the Mediterranean may have closed due to tearing and sinking of the lithosphere.

Looking for evidence to support this theory (called the Zanclean flood hypothesis), an international group of researchers recently identified buried sediments that could have been deposited by the flooding waters. "One of the recurrent questions was 'Where did all the sediments accumulate after the flood?'" said Daniel Garcia-Castellanos, a geophysicist at the Institute of Earth Sciences Jaume Almera in Barcelona, Spain, who led the search and has studied the Zanclean flood hypothesis over the past decade.

Using computer simulations to recreate the opening of the strait and the subsequent flood, Garcia-Castellanos and his team identified low-flow areas that were likely to accumulate sediments. Then, they looked at seismic profiles of these areas to see if they could find the sediments.

A composite seismic profile shows the Messinian erosion surface (MSC, purple line) on the eastern side of the Strait of Gibraltar. This unconformity is interpreted as the erosion channel excavated into Miocene sediments during late Messinian Salinity Crisis or earliest Pliocene. Credit: Garcia-Castellanos et. al., CC BY 4.0

On the lee side of an ancient volcanic cone in the predicted path of the flooding, they found a large sediment deposit. It appears as an amorphous blob in the seismic profile, in contrast to the neatly stratified layers of marine sediments around it, suggesting rapid sedimentation. Its shape and size also seem to match the likely direction of the flood.

"This shape is compatible with the numerical models and with what we see in other megaflood settings," Garcia-Castellanos said.

In 2018, the same group of researchers found similar deposits near a deep underwater gorge in the Malta Escarpment, the natural barrier separating the eastern and western Mediterranean basins. This gorge, known as the Noto Canyon, is the most likely location where the flood might have spilled over the escarpment to refill the eastern Mediterranean basin.

The main limitation of these simulations is the lack of knowledge about ancient seafloor topography. "We used the present-day seafloor morphology since we don't know how it was in the past," explained Garcia-Castellanos. "Nevertheless, these simulations give us a quantitative idea of how far can such a flow transport each grain size."

The researchers published their findings in a paper reviewing the Zanclean flood hypothesis in Earth-Science Reviews.

To Flood or Not to Flood

The Zanclean flood hypothesis has its detractors. Many researchers studying the MSC are not even sure the Mediterranean actually desiccated much below its modern-day level.

One of the main problems with the desiccation theory is the utterly massive amounts of salt and gypsum thought to have been deposited during the MSC. If its waters had instantly evaporated, the present-day Mediterranean Sea would leave behind a layer of salt no thicker than 30 meters. In comparison, the salt layers associated with the MSC are up to a kilometer thick in some areas.

To accumulate so much salt, critics of the Zanclean hypothesis say, water had to be able to flow into the sea from the open ocean, albeit in a restricted way that could limit the escape of the denser, briny water. This dense water would sink to the deepest parts of the sea, where minerals could precipitate.

"This debate won't end tomorrow."Another controversial line of evidence is the networks of ancient channels thought to be the remains of ancient river systems. Current research suggests that these channels could have been carved by the flow of dense, salty water as it sank to the deepest areas of the sea. A similar effect could have formed the erosion channel at the bottom of the Strait of Gibraltar.

Looking for a different way to assess the problem, Javier Garcia Veigas, a researcher at the University of Barcelona not involved in the new research, looked at the isotopic signatures of Messinian gypsum deposits at several points in the Mediterranean. Biological activity and other factors can change the ratios of heavy to light isotopes of certain chemical species in seawater. These differences should be measurable in the minerals formed in aquatic environments with different isotopic compositions. However, Garcia Veigas didn't find significant isotopic differences between rocks formed in the eastern and western Mediterranean basins, suggesting that they remained connected.

"I think that the consensus in the community is slowly drifting away from the megaflood hypothesis and increasingly moving towards a scenario where the Mediterranean did not fully desiccate," Garcia Veigas said. "However, we won't have definitive proof until a drilling campaign can reach the deep Mediterranean basin and confirm that what we see in the seismic profile is really salt formed during the Messinian."

Garcia-Castellanos acknowledges that not everybody is on board with the Zanclean flood hypothesis. "All of the evidence that has been summarized in this article may have other possible interpretations," he said. "So this debate won't end tomorrow."

Javier Barbuzano (@javibarbuzano), Freelance Science Journalist

Six million years ago, the Mediterranean Sea was cut off from the Atlantic Ocean. The collision of the African and Eurasian tectonic plates caused the uplift of the Rif and Betic Mountains (in northern Morocco and southern Spain, respectively), closing at least two large straits or channels that connected the water bodies at the time.

六百万年前,地中海和大西洋被切断。非洲板块和欧亚板块的碰撞导致里夫山脉和贝提克山脉(分别位于摩洛哥北部和西班牙南部)的抬升,至少封闭了两个连接水体的大海峡。

This event caused a sudden increase in salinity in the Mediterranean, known as the Messinian Salinity Crisis (MSC), which in turn led to the formation of massive gypsum and salt deposits throughout the Mediterranean.

这一事件导致了地中海盐度的突然增加,被称为Messinian盐度危机(MSC),这又导致了地中海大量石膏和盐沉积物的形成。

Scientists are still debating about what happened to sea levels in the Mediterranean after its western waterways closed. One possibility is that water evaporated quickly and sea levels fell by thousands of meters. Only a series of salty or brackish lakes might have survived. Eroded channels and sediment deposits in what might have been the ancient surface of the exposed Mediterranean basin seem to support this theory.

科学家们仍在争论关闭地中海西部水道后,地中海的海平面究竟发生了什么变化。一种可能性是水迅速蒸发,海平面下降数千米。只有一些咸水或半咸水湖泊可能幸存下来。在可能是地中海盆地古表面的侵蚀河道和沉积物似乎支持这一理论。

Zanclean Flood Hypothesis

Zanclean洪水假说

The period of the dry Mediterranean might have ended quickly when the Strait of Gibraltar reopened 5.33 million years ago, causing a massive flood that refilled the empty basin in just 2 years. Seismic reflection imaging, a technique that relies on sound waves to image what lies underground, has revealed that beneath kilometer-deep Quaternary sediments, a huge erosion channel extends across the strait. The ancient channel, several hundred meters deep and 390 kilometers long, stretches from the Gulf of Cdiz (in the Atlantic) to the Algerian Basin (in the Mediterranean). Researchers think that this channel might have been carved during the flood as Atlantic waters rushed in to refill the Mediterranean.

当直布罗陀海峡在533万年前重新开放时,干旱的地中海时期可能很快就结束了,这导致了一场巨大的洪水,仅用两年时间就将这个干涸的盆地填满。地震反射成像技术是一种依靠声波来成像地下的技术,它揭示了在几千米深的第四系沉积物下,一个巨大的侵蚀通道横跨海峡。这条古老的海峡有几百米深,390公里长,从大西洋的Cdiz湾一直延伸到地中海的阿尔及利亚盆地。研究人员认为,这条水道可能是在大西洋洪水涌入地中海时开凿的。

These two videos summarize our interpretation of what happened to the Mediterranean ~5 million years ago based on geological & geophysical data. First, the connections between the Atlantic and the Mediterranean may have closed due to tearing and sinking of the lithosphere.

这两个视频总结了我们基于地质和地球物理数据对500万年前地中海发生的事情的解释。首先,由于岩石圈的撕裂和下沉,大西洋和地中海之间的连接可能已经关闭。

Looking for evidence to support this theory (called the Zanclean flood hypothesis), an international group of researchers recently identified buried sediments that could have been deposited by the flooding waters. "One of the recurrent questions was 'Where did all the sediments accumulate after the flood?'" said Daniel Garcia-Castellanos, a geophysicist at the Institute of Earth Sciences Jaume Almera in Barcelona, Spain, who led the search and has studied the Zanclean flood hypothesis over the past decade.

为了寻找支持这一理论的证据(称为赞克林洪水假说),一个国际研究小组最近发现了可能是洪水沉积的地下沉积物。“一个经常出现的问题是‘洪水过后,所有的沉积物都堆积在哪里?’”’”西班牙巴塞罗纳Jaume Almera地球科学研究所的地球物理学家Daniel Garcia-Castellanos说。

Using computer simulations to recreate the opening of the strait and the subsequent flood, Garcia-Castellanos and his team identified low-flow areas that were likely to accumulate sediments. Then, they looked at seismic profiles of these areas to see if they could find the sediments.

加西亚-卡斯特利亚诺斯和他的团队利用计算机模拟重现了海峡的开口和随后的洪水,确定了可能堆积沉积物的低流量区域。然后,他们查看这些地区的地震剖面,看看是否能找到沉积物。

A composite seismic profile shows the Messinian erosion surface (MSC, purple line) on the eastern side of the Strait of Gibraltar. This unconformity is interpreted as the erosion channel excavated into Miocene sediments during late Messinian Salinity Crisis or earliest Pliocene. Credit: Garcia-Castellanos et. al., CC BY 4.0

复合地震剖面显示了位于直布罗陀海峡东侧的墨西尼侵蚀面(MSC,紫色线)。这种不整合被解释为在晚墨西期盐化危机或上新世早期在中新世沉积物中开挖的侵蚀通道。来源:Garcia-Castellanos等。阿尔,抄送4

On the lee side of an ancient volcanic cone in the predicted path of the flooding, they found a large sediment deposit. It appears as an amorphous blob in the seismic profile, in contrast to the neatly stratified layers of marine sediments around it, suggesting rapid sedimentation. Its shape and size also seem to match the likely direction of the flood.

在一个古代火山锥的背风面,在洪水的预测路径中,他们发现了一个巨大的沉积物沉积。在地震剖面上,它以无定形的斑点状出现,与周围整齐的分层的海洋沉积层形成对比,表明沉积速度很快。它的形状和大小似乎也与洪水可能的方向相匹配。

"This shape is compatible with the numerical models and with what we see in other megaflood settings," Garcia-Castellanos said.

加西亚-卡斯特利亚诺斯说:“这种形状与数值模型和我们在其他大洪水环境中看到的形状是一致的。”

In 2018, the same group of researchers found similar deposits near a deep underwater gorge in the Malta Escarpment, the natural barrier separating the eastern and western Mediterranean basins. This gorge, known as the Noto Canyon, is the most likely location where the flood might have spilled over the escarpment to refill the eastern Mediterranean basin.

2018年,同一组研究人员在马耳他悬崖的水下峡谷附近发现了类似的沉积物。马耳他悬崖是分隔地中海东部和西部盆地的天然屏障。这个峡谷,被称为Noto峡谷,是洪水最有可能漫过悬崖填满地中海东部盆地的地方。

The main limitation of these simulations is the lack of knowledge about ancient seafloor topography. "We used the present-day seafloor morphology since we don't know how it was in the past," explained Garcia-Castellanos. "Nevertheless, these simulations give us a quantitative idea of how far can such a flow transport each grain size."

这些模拟的主要局限性是缺乏有关古代海底地形的知识。加西亚-卡斯特利亚诺斯解释说:“我们使用了现在的海底形态,因为我们不知道过去是怎样的。”“然而,这些模拟给了我们一个定量的概念,即这样的气流可以将每种颗粒大小输送到多远的地方。”

The researchers published their findings in a paper reviewing the Zanclean flood hypothesis in Earth-Science Reviews.

研究人员在《地球科学评论》上发表了一篇关于赞克尔洪水假说的论文。

To Flood or Not to Flood

洪水还是不洪水

The Zanclean flood hypothesis has its detractors. Many researchers studying the MSC are not even sure the Mediterranean actually desiccated much below its modern-day level.

赞斯尔洪水假说也有批评者。许多研究地中海的学者甚至不确定地中海的实际干化程度是否远低于现代水平。

One of the main problems with the desiccation theory is the utterly massive amounts of salt and gypsum thought to have been deposited during the MSC. If its waters had instantly evaporated, the present-day Mediterranean Sea would leave behind a layer of salt no thicker than 30 meters. In comparison, the salt layers associated with the MSC are up to a kilometer thick in some areas.

干燥理论的主要问题之一是在MSC期间沉积了大量的盐和石膏。如果海水立即蒸发,现在的地中海就会留下一层不超过30米厚的盐。相比之下,与MSC相关的盐层在一些地区厚达一公里。

To accumulate so much salt, critics of the Zanclean hypothesis say, water had to be able to flow into the sea from the open ocean, albeit in a restricted way that could limit the escape of the denser, briny water. This dense water would sink to the deepest parts of the sea, where minerals could precipitate.

赞克尔假说的批评者说,要积累这么多的盐,水必须能够从开阔的海洋流入大海,尽管是以一种受限的方式,可能会限制密度更大的咸水的流出。这些稠密的水会下沉到海洋的最深处,在那里矿物质会沉淀。

"This debate won't end tomorrow."Another controversial line of evidence is the networks of ancient channels thought to be the remains of ancient river systems. Current research suggests that these channels could have been carved by the flow of dense, salty water as it sank to the deepest areas of the sea. A similar effect could have formed the erosion channel at the bottom of the Strait of Gibraltar.

“这场辩论不会在明天结束。有争议的证据是古代河道网络被认为是古代河流系统的遗迹。目前的研究表明,这些水道可能是在海水沉入海底最深处时,由密度大的咸水冲刷而成的。类似的影响可能在直布罗陀海峡底部形成侵蚀通道。

Looking for a different way to assess the problem, Javier Garcia Veigas, a researcher at the University of Barcelona not involved in the new research, looked at the isotopic signatures of Messinian gypsum deposits at several points in the Mediterranean. Biological activity and other factors can change the ratios of heavy to light isotopes of certain chemical species in seawater. These differences should be measurable in the minerals formed in aquatic environments with different isotopic compositions. However, Garcia Veigas didn't find significant isotopic differences between rocks formed in the eastern and western Mediterranean basins, suggesting that they remained connected.

巴塞罗那大学(University of Barcelona)的研究员哈维尔·加西亚·维加斯(Javier Garcia Veigas)没有参与这项新研究,他在寻找一种不同的方法来评估这个问题。生物活性等因素会改变海水中某些化学物质重同位素与轻同位素的比例。这些差异在不同同位素组成的水环境中形成的矿物中是可以测量的。然而,加西亚·维加斯并没有发现东地中海盆地和西地中海盆地形成的岩石之间存在明显的同位素差异,这表明它们之间仍然存在联系。

"I think that the consensus in the community is slowly drifting away from the megaflood hypothesis and increasingly moving towards a scenario where the Mediterranean did not fully desiccate," Garcia Veigas said. "However, we won't have definitive proof until a drilling campaign can reach the deep Mediterranean basin and confirm that what we see in the seismic profile is really salt formed during the Messinian."

加西亚·维加斯说:“我认为社区的共识正在慢慢地偏离大洪水假说,并逐渐向地中海没有完全干涸的情况发展。”“然而,在钻探活动到达地中海深处盆地并确认我们在地震剖面上看到的确实是在Messinian期间形成的盐之前,我们还没有确凿的证据。”

Garcia-Castellanos acknowledges that not everybody is on board with the Zanclean flood hypothesis. "All of the evidence that has been summarized in this article may have other possible interpretations," he said. "So this debate won't end tomorrow."

加西亚-卡斯特利亚诺斯承认,并不是所有人都认同赞克利洪水假说。“这篇文章中总结的所有证据都可能有其他的解释,”他说。“所以这场辩论不会在明天结束。”

Javier Barbuzano (@javibarbuzano), Freelance Science Journalist

Javier Barbuzano (@javibarbuzano),自由科学记者