华南二叠纪-三叠纪之交海相碳酸盐岩成岩记录研究

It has long been thought that the pervasive synchronic changes in the δ13C of marine carbonates (δ13Ccarb) reflect changes in global carbon cycles that may be related to important geological events such as mass extinctions. Recent studies of Pliocene-Pleistocene limestones found on isolated oceanic islands, however, indicate that the synchronic changes in δ13Ccarb may be caused by diagenesis rather than changes in the global carbon cycle. These conflicting notions need to be assessed by examining other carbonate successions. This study will focus on the δ13Ccarb in the carbonate rocks that are associated with the Permian-Triassic mass extinction boundary in South China. This negative δ13Ccarb is found in sections throughout the world, while the Pliocene-Pleistocene limestone successions on oceanic islands also display synchronic negative δ13Ccarb. In order to determine the influence of diagenesis on the negative δ13Ccarb in the carbonate rocks at the Permian-Triassic boundary, this study will (1) compare the variation of δ13Ccarb between the P-T intervals from South China with the Pliocene-Pleistocene limestone succession on Grand Cayman, (2) apply the results obtained from the Grand Cayman limestone succession to examine the diagenetic features of the Permian-Triassic carbonate succession, and (3) assess the degree of alteration in the mineralogy and geochemical features caused by diagenesis. This study will improve our knowledge about the impact of diagenesis on the synchronic changes in δ13C of marine carbonates (δ13Ccarb) associated with important geological events.

与重大地质历史事件相伴的海相碳酸盐岩无机碳同位素(δ13Ccarb)全球同步变化常被认为是全球碳循环变化引起的。但是, 对孤立洋岛上年轻碳酸盐岩地层的研究发现, 全球同步变化的δ13Ccarb负偏是成岩作用改造引起的, 与全球碳循环变化无关。为探讨二叠纪-三叠纪之交海相碳酸盐岩中具全球同步变化特征的δ13Ccarb负偏是否受到成岩作用的影响, 本项目拟采用将今论古的方法, 比对并查找Grand Cayman岛屿更新世灰岩序列与该古老碳酸盐岩地层无机碳同位素变化的共异性, 将Grand Cayman岛屿更新世灰岩成岩作用的研究成果应用到二叠纪-三叠纪界线海相碳酸盐岩, 考察其成岩作用过程并评估成岩作用对矿物和地球化学特征(特别是δ13Ccarb)的改造程度。该研究成果为进一步认识重大地质历史转折时期海相碳酸盐岩中无机碳同位素全球同步变化现象的成因提供了新的视角。

二叠纪-三叠纪之交(PTB)海相碳酸盐岩的无机碳同位素表现有明显的负偏现象，被前人解释为全球碳循环变化所致。这一解释的理论基础是测得的无机碳同位素值记录的是海相碳酸盐岩沉积时期的信号。但是，对年轻(上新世-更新世)碳酸盐岩地层和新元古代碳酸盐岩地层的研究发现，大气淡水成岩作用和埋藏成岩作用也可以导致同步的无机碳同位素负偏现象。在我国华南地区，二叠纪最末期发生了广泛的海平面下降事件，二叠系-三叠系界线地层序列也经历过长期的埋藏过程，因此，本研究提出猜想：二叠系-三叠系界线处的无机碳同位素负偏很可能与海平面下降所引起的不整合面及相伴的大气淡水成岩作用或埋藏成岩作用相关。为验证这一猜想，本项目重点对全球二叠系-三叠系界线层型剖面煤山剖面D的24层-62层、二叠系-三叠系之交台地相剖面(江西沿沟剖面、湖南慈利剖面、广西太平剖面)中相当于煤山剖面24层-29层的碳酸盐岩开展了成岩作用的研究工作，并探讨了成岩作用对无机碳同位素负偏的贡献量。研究发现(1)与煤山剖面26层和28层相当层位中的δ13C负偏由大气淡水成岩作用引起，而白云石化地层中的δ13C负偏是同沉积白云石形成后遭后期成岩改造作用引起的，可导致1.7‰的负偏。(2)白云石晶体呈环带结构，低钙钙白云石(LCD)核部依次由不规则状方解石环带，高钙钙白云石(HCD)环带和最外层的富铁高钙钙白云石(Fe-HCD)环带组成，说明白云石为多幕次成岩作用产物。 LCD核部和HCD环带在同沉积时期形成后，经历了大气淡水作用下的去白云石化作用和埋藏过程中的富铁HCD的增生。(3) 如果剔除成岩作用对δ13C的影响,台地至斜坡相海水的无机碳同位素深度梯度小于之前报道的数据。研究结果表明PTB地层的无机碳同位素具有成岩记录信号。