NOAA/WDS Paleoclimatology - Lake Malawi 1.2 Million Year Ostracod and Geochemical Lake Level Data

Long paleoecological records are critical for understanding evolutionary responses to environmental forcing and unparalleled tools for elucidating the mechanisms that lead to the development of regions of high biodiversity. We use a 1.2-My record from Lake Malawi, a textbook example of biological diversification, to document how climate and tectonics have driven ecosystem and evolutionary dynamics. Before ~800 ka, Lake Malawi was much shallower than today, with higher frequency but much lower amplitude water-level and oxygenation changes. Since ~800 ka, the lake has experienced much larger environmental fluctuations, best explained by a punctuated, tectonically driven rise in its outlet location and level. Following the reorganization of the basin, a change in the pacing of hydroclimate variability associated with the Mid-Pleistocene Transition resulted in hydrologic change dominated by precession rather than the high-latitude teleconnections recorded elsewhere. During this time, extended, deep lake phases have abruptly alternated with times of extreme aridity and ecosystem variability. Repeated crossings of hydroclimatic thresholds within the lake system were critical for establishing the rhythm of diversification, hybridization, and extinction that dominate the modern system. The chronology of these changes closely matches both the timing and pattern of phylogenetic history inferred independently for the lake's extraordinary array of cichlid fish species, suggesting a direct link between environmental and evolutionary dynamics.

长期古生态记录是理解生物对环境驱动因子演化响应的关键载体，同时也是阐明高生物多样性区域形成机制的无与伦比的研究工具。我们以作为生物多样化经典研究案例的马拉维湖（Lake Malawi）的1.2-My记录为依托，解析气候与构造活动如何驱动了生态系统与演化动态。在约80万年前（~800 ka），马拉维湖的水深远低于现今，其时水位与氧合作用变化的频率更高，但幅度远更小。自约80万年前以来，该湖经历了幅度大得多的环境波动，这一现象最佳的解释为：其出水口位置与高程出现了间断性的构造抬升。在流域重组之后，与中更新世过渡期（Mid-Pleistocene Transition）相关的水文气候变率节律发生改变，使得水文变化以岁差（precession）驱动为主，而非其他区域所记录的高纬度遥相关模式。在此期间，持续的深水湖阶段与极端干旱及生态系统波动时期发生了骤然交替。湖系统内水文气候阈值的反复跨越，对于确立主导现代生态系统的多样化、杂交与灭绝节律起到了关键作用。这些变化的时间序列，与该湖特有的慈鲷鱼类（cichlid fish）类群独立推断出的系统发育历史的时间与模式高度吻合，表明环境动态与演化动态之间存在直接关联。