更新世冰盖与大洋碳储库相互作用的箱式模型模拟

Interaction between Ice Sheet and Oceanic Carbon Cycling during the Pleistocene: A Box Model Simulation

全球大洋深海有孔虫碳同位素(δ13C)记录中广泛发现40万年周期,这一周期可能与偏心率长周期的轨道驱动有关.1.6 Ma以来,δ13C的这一长周期拉长到50万年,且重值期不再与偏心率低值对应.目前对δ13C 40万年周期的成因及其周期拉长的机制还不明确.这里使用了包含9个箱体的箱式模型,用于研究热带过程与冰盖相互作用及其对大洋碳循环的影响.模拟结果显示当北半球高纬海区海冰迅速增大时冰盖迅速融化,进入冰消期,而当海冰快速消失后,冰盖则重新缓慢增长.冰盖变化具有冰期长,间冰期短的非对称形态.在季节性太阳辐射量的驱动下冰盖变化具有10万年冰期-间冰期旋回.当冰盖融化速率受北半球高纬夏季太阳辐射量控制时,冰盖变化的岁差周期明显加强,相位与地质记录一致,说明轨道驱动可以通过非线性相位锁定机制使冰盖变化与其在相位上保持一致.海冰的阻隔效应使大气中CO2在冰消期时增多.冰期时大洋环流减弱使大气中CO2逐渐减少.当模型只有ETP驱动的风化作用而不考虑冰盖变化时,模拟的δ13C记录显示极强的40万年周期,体现了大洋碳储库对热带风化过程的响应.当同时考虑冰盖变化和风化作用时,模拟的δ13C结果中40万年周期减弱而10万年周期加强,并且40万年周期上碳储库与偏心率的相位与不考虑冰盖变化时的相位也存在差异,反映了冰盖变化引起的洋流改组压制了大洋碳循环对热带过程的响应.

The widely discovered 400-kyr cycles of foraminiferal carbon isotopes （δ^13 C） from world oceans are interpreted to be linked to the forcing of Earth＇s eccentricity around the Sun. During the past 1.6 million years （Ma）, however, this period extended to 500-kyr and the δ^13C maxima of the carbon cycle didn＇t correspond to the eccentricity＇s minima. The origin of the 400- kyr cycle and the mechanism for its obscuring during the Pleistocene are elusive. Here we develop a 9 box biogeochemical model on the purpose of understanding interactions between tropical process and variability of ice sheet and their influences on oceanic carbon cycle. The simulated results show that deglaciation is concurrent with the appearance of sea ice in the northern high latitude; while when the northern high latitude is free of sea ice, the ice sheet begins to build up. The simulated building of ice sheet is slower than its retreat, thus the variability of ice sheet is asymmetric. The model can simulate asyrmnetric 100 kyr cycle of ice sheet under only seasonal solar insolation rather than Milankovitch forcing. By adding the summer insolation of northern high latitude into the ablation term of the ice sheet, precession components become stronger in simulated results. The onset of deglaciation is nonlinearly phase locked to the summer insolation forcing, which leads to a good comparison with geological record. The simulated concentration of atmospheric CO2 is becoming higher during deglaciation owing to the insulation effect of sea ice and is becoming lower during the decrease of ocean circulation. If the model is forced by only tropical weathering process but without the variability of ice sheet, simulated δ^13 results exhibit strong 400-kyr cycles indicating a significant response of ocean carbon reservoir to tropical forcing. If the model is forced by both the variability of weathering and ice sheet, however, simulated δ^13 results show weaker 400 kyr but stronger lO0-kyr cycles and the phase difference between the simulated δ^13 and the eccentricity at the 400-kyr band is also different from that in the simulation forced by only weathering process. Our model results seem to indicate that when ice sheet is introduced into the earth system it will result in oceanic circulation reorganization which can suppress the signal of tropical process in ocean carbon reservoir.