Mount Toba eruption doesn’t seem like it could nearly kill our species

多巴火山爆发似乎并未导致人类濒临灭绝

Mount Toba was the biggest volcanic eruption in the last 2.6 million years, and some have suggested it nearly wiped out humanity. Around 74,000 years ago, a caldera on what is now Sumatra emptied thousands of cubic kilometers of magma in about two weeks, roughly a thousand times more than Mount Pinatubo spewed out in 1991. 多巴火山（Mount Toba）是过去260万年来规模最大的火山喷发，一些人认为它曾导致人类濒临灭绝。大约7.4万年前，位于现今苏门答腊岛的一个火山口在两周左右的时间里喷发了数千立方公里的岩浆，其规模大约是1991年皮纳图博火山（Mount Pinatubo）喷发量的1000倍。

“People thought it might have caused massive cooling of the planet, and hence threatened the survival of our ancestors,” says Jinheum Park, a geoscientist at Johannes Gutenberg University in Mainz, Germany. Park and his colleagues went looking for the records of that catastrophe in the mud taken from the bottom of a small crater lake on the Kenya-Tanzania border. They found instead that the effects of the Mount Toba eruption lasted under two years and amounted to perhaps half a degree of cooling. 德国美因茨约翰内斯·古腾堡大学的地球科学家朴金赫（Jinheum Park，音译）表示：“人们曾认为这可能导致了全球气温大幅下降，从而威胁到我们祖先的生存。”朴和他的同事们在肯尼亚与坦桑尼亚边境的一个小型火山口湖底淤泥中寻找这场灾难的记录。然而他们发现，多巴火山喷发的影响持续时间不到两年，气温降幅可能仅为半摄氏度左右。

Muddy calendars

泥泞的日历

Volcanic eruptions inject sulfur dioxide into the stratosphere, where it becomes a haze of tiny droplets that reflect sunlight back into space. Bigger eruptions eject more sulfur dioxide and, in principle, cause more cooling. But this trend stops working when the eruption exceeds a certain magnitude. “Bigger sulfate aerosols settle quickly, because they are heavier,” Park explains. This quick settling makes them less effective in scattering incoming solar radiation. 火山喷发会将二氧化硫注入平流层，形成一层微小液滴组成的薄雾，将阳光反射回太空。喷发规模越大，喷出的二氧化硫就越多，理论上会导致更明显的降温。但当喷发规模超过一定限度时，这种趋势就会失效。朴解释说：“较大的硫酸盐气溶胶因为更重，沉降得更快。”这种快速沉降使其在散射入射太阳辐射方面的效果大打折扣。

Estimates of Toba’s sulfur output varied so widely that computer models had the eruption appearing as anything between a near-extinction-level event to a mild nuisance for our early ancestors, depending on the number we punched in. To settle this argument, scientists tried to look at geological evidence: Toba’s ash preserved in seafloor or lake cores. But this also proved tricky. Underwater mud cores usually don’t work that well when it comes to dating and constraining abrupt, violent events like volcanic eruptions. 关于多巴火山硫排放量的估算差异巨大，以至于计算机模型显示，根据输入数值的不同，这次喷发对我们早期祖先的影响可能从“近乎灭绝的灾难”到“轻微的困扰”不等。为了解决这一争议，科学家们试图寻找地质证据：保存在海底或湖泊岩芯中的多巴火山灰。但这也被证明很棘手。在测定和限定火山喷发这类突发性、剧烈事件的时间时，水下泥芯通常效果不佳。

When sediment settles at the lake or ocean bottom, materials from different years get a bit mixed. Even if we cut a mud core taken from the bottom of the sea into very thin slices and attempt to resolve climate changes at smaller intervals, our slices would reflect the mixed influence of yearslong climate signals. With most such records, we can track how past climates evolved decade by decade. A volcanic winter, even a severe one, would last one to three years. “It’s really difficult to find any sedimentary archive which registers the regional climate at such high resolution,” Park says. Looking for traces of a Toba catastrophe in standard sediment cores was like timing a sprint with a calendar. But then scientists found a lake with sediment records that worked more like a stopwatch. 当沉积物在湖底或海底沉降时，不同年份的物质会发生混合。即使我们将海底泥芯切成极薄的切片，试图以更小的时间间隔解析气候变化，这些切片反映的也只是多年气候信号的混合影响。通过大多数此类记录，我们只能追踪过去气候以十年为单位的演变。而火山冬天，即使是严重的火山冬天，也只会持续一到三年。朴说：“很难找到任何能以如此高分辨率记录区域气候的沉积档案。”在标准的沉积岩芯中寻找多巴灾难的痕迹，就像用日历来计时短跑比赛。但随后，科学家们发现了一个沉积记录更像秒表的湖泊。

Stopwatch lake

秒表湖

Lake Chala is a small, steep-walled crater lake on the flank of Mount Kilimanjaro, fed by groundwater from the mountain’s forested slopes. Its deep water never fully turns over, leaving the bottom starved of oxygen and free of currents. This makes it a nearly perfect hi-res climate recorder, in which sediments are deposited as varves: annual couplets with a light-colored layer rich in silica skeletons from algae called diatoms and a dark-colored layer with fine-grained soils and clays. “It works just like tree rings,” Park says. 查拉湖（Lake Chala）是乞力马扎罗山侧翼一个陡峭的小型火山口湖，由山坡森林地带的地下水补给。其深层水体从未完全翻转，导致湖底缺氧且没有水流。这使其成为一个近乎完美的高分辨率气候记录仪，沉积物以纹泥（varves）形式堆积：即由富含硅藻（一种藻类）硅质骨架的浅色层，以及含有细颗粒土壤和粘土的深色层组成的年度对偶层。朴说：“它就像树木年轮一样。”

The couplets exist because from October through April, the water stays layered and still as the dark materials accumulate. Then, in the cool, dry, windy months from June to September, it mixes. “The lake is 90 meters deep, but during cool and dry conditions the water can be mixed down to about 50 meters,” Park explains. “In that case, the nutrients near the lake bottom come up to the surface, and then the diatoms can make use of them.” Diatoms, fed by that upwelling material, bloom, die, and sink as a pale layer of silica—a layer that’s thicker in cool, dry years when the mixing season runs long. 这些对偶层的存在是因为从10月到次年4月，水体保持分层且静止，深色物质在此期间堆积。随后，在6月到9月凉爽、干燥且多风的月份，水体发生混合。朴解释说：“该湖深90米，但在凉爽干燥的条件下，水体可以混合到约50米深。在这种情况下，湖底附近的营养物质会上升到表面，硅藻便能利用它们。”以这些上升物质为食的硅藻会大量繁殖、死亡，并沉降形成一层浅色的硅质层——在混合季节较长的凉爽干燥年份，这一层会更厚。

Invisible ash

隐形的火山灰

Toba’s ash at Chala was identified by earlier work. It is invisible to the naked eye, its particles too sparse and small to form a visible layer. Scientists found them by dissolving away the mud and counting microscopic glass shards, remnants of the molten magma the eruption atomized into ultra-fine and rapidly cooling dust. At Chala the shards spike abruptly, then vanish. “There is a sharp increase and a sharp peak, so we could rule out any significant influence by redeposition,” Park says. “We were quite sure that it was from direct ash fall.” 早期的研究已经在查拉湖发现了多巴火山灰。它肉眼不可见，颗粒过于稀疏且细小，无法形成可见的层。科学家们通过溶解淤泥并计数微小的玻璃碎片发现了它们，这些碎片是喷发时被雾化成超细且快速冷却的尘埃的熔岩残余。在查拉湖，这些碎片的数量急剧增加，然后消失。朴说：“有一个明显的增长和峰值，所以我们可以排除再沉积带来的重大影响。我们非常确定这是直接的火山灰沉降。”

In X-ray scans, the layer is a sliver 0.3 millimeters thick—thinner than a sheet of paper. Park’s team then worked through the 450 years of mud bracketing that sliver, layer by layer, combining microscope images with chemical scans, isotope readings, and diatom counts every two to three years. For the 260 years before the ash fell, the team found that the lake was calm year-round, and the climate was warm and wet. Then comes the ash and, sandwiched inside and just above it, two green films a few hundredths of a millimeter thick. 在X射线扫描中，该层是一个0.3毫米厚的薄片，比一张纸还要薄。朴的团队随后对该薄片前后450年的淤泥进行了逐层研究，将显微镜图像与化学扫描、同位素读数以及每两到三年的硅藻计数相结合。在火山灰落下前的260年里，研究小组发现湖泊全年平静，气候温暖湿润。随后是火山灰，在火山灰内部及其上方，夹着两层几百分之一毫米厚的绿色薄膜。

“We couldn’t exactly identify what those green layers were, but at least it was not from incompletely decomposed algae, because we couldn’t find any intact remains,” Park says. The team interpreted these layers as material diatoms secrete when they are badly stressed. “We thought the green layers came from the diatoms, as a stress response to the dimmed sky,” Park says. “Because they need light to grow.” 朴说：“我们无法确切识别这些绿色层是什么，但至少它们不是来自未完全分解的藻类，因为我们找不到任何完整的残骸。”研究小组将这些层解释为硅藻在受到严重压力时分泌的物质。朴说：“我们认为这些绿色层来自硅藻，是对天空变暗的一种压力反应，因为它们需要光线才能生长。”

The next dry season after the eruption produced a pale layer 1.2 millimeters thick, with abundant diatoms—a bloom apparently driven by the unusually deep, prolonged mixing a chilled lake surface would cause. The dark layer above it is thin and faint, meaning the rains that followed were weak, probably due to a cooled Indian Ocean sending less moisture inland. But by the third year, the Chala layers are back to normal. “The light lamina that was deposited nearly two years after the eruption is quite typical for Lake Chala,” Park says. 喷发后的下一个旱季产生了一层1.2毫米厚的浅色层，其中含有大量的硅藻——这种繁盛显然是由湖面冷却导致的异常深且持久的混合所驱动的。其上方的深色层又薄又淡，意味着随后的降雨很弱，这可能是因为冷却的印度洋向内陆输送的水汽减少了。但到了第三年，查拉湖的沉积层已恢复正常。朴说：“在喷发近两年后沉积的浅色薄层，对于查拉湖来说是非常典型的。”