陆相坳陷湖盆沉积对深时古气候的响应信号:以准噶尔盆地西北缘安集海河组为例

"深时"古气候研究范围为前第四纪地质记录,目的在于为未来气候预测提供依据.准噶尔盆地西北缘安集海河组沉积期,研究区处于早古近纪极热气候(PETM)时期,由于其温室气体排放与当前工业废气排放水平相当,对于全球变化以及地球系统科学研究具有重要意义,一直以来都是古近纪气候研究的热点.准噶尔盆地西北缘在早古近纪沉积期发育大范围...

Signals of depositional response to the deep time paleoclimate in continental depression lakes:Insight from the Anjihaihe Formation in the northwestern Junggar Basin

The deep time paleoclimate study concerns with the pre-Quaternary geological records and aims at predicting climate change in the future. In the northwestern Junggar Basin, the Anjihaihe Formation period coincided with the Paleocene-Eocene Thermal Maximum (PETM), an abrupt warming event in the early Paleogene and one of the most pronounced transient climatic events in the geological record. This warming event was likely related to the greenhouse gas emission from seafloor gas-hydrate accumulations. On a millennial timescale, the rate of greenhouse gas emission is, to large extend, comparable to current industrial events, thus providing a unique insight into our uncertain climate future. Therefore, this event has always been a hotspot in Paleogene climate research. During the early Paleogene, typical basin lake deposits were widely distributed in the northwestern Junggar Basin. However, the much smaller scale lacustrine sediments were much more sensitive to climate change than the oceanic system, thus providing more accurate recordings of the paleoclimate evolution that controlled the sedimentary system of the lake basin. In this study, we selected 28 sandstone and 23 mudstone samples from the Anjihaihe Formation for the grain-size analysis and trace element testing, respectively. The results showed that the median diameter of the sandstone grains varied from 7.58 to 393 μm, averaging at 74.44 μm and belonging to fine sand. Furthermore, the enrichments of Ni, Cu and Zn indicated high paleoproductivity during the Anjihaihe Formation period; the Mo, Re, V, Co and Cr contents and Re/Mo, V/Sc, V/Cr and Ni/Co ratios demonstrated the study area was developed in an oxidizing environment; and the Sr/Cu ratio suggested dry and hot climate. We next studied comprehensively the response of continental depression lake to the PETM using core observation and microscopic identification methods, searching response signals from geological archives of both sedimentary and biogeochemical records. Under the conditions of the PETM, the entire study area was in a hot and dry climate. As drought condition intensified, intense evaporation caused lake level to fall. As a result, sediment grain size gradually coarsened, and exposed sediment near the water surface was oxidized to form expansive continental red beds. Meanwhile, due to increasing partial pressure of CO₂ in lake water, supersaturated carbonate minerals precipitated in the form of basement carbonate cement. Continuing concentration of lake water also caused rising salinity, creating a continental brackish water sedimentary environment. Oxidability of water increased too, whereas paleoproductivity increased first and then decreased.