NOAA/WDS Paleoclimatology - Global change in deep sea carbon isotopes during the mid-Pleistocene transition

Five δ13C records from the deep ocean, extending back to 1.3 Ma, were examined in order to constrain changes in mean ocean carbon isotope composition and thermohaline circulation over the 41- to 100-ka climate transition. These data show that significant perturbations in mean ocean carbon chemistry were associated with the mid-Pleistocene climate transition. Notable features of the last 1.3 Myr are (1) a pronounced ∼0.3‰ decrease in mean ocean δ13C between 0.9 and 1.0 Myr, followed by a return to pre-1.0 Ma values by 400 ka B.P., which we propose was due to the onetime addition of isotopically depleted terrestrial carbon to the ocean, possibly associated with an increase in global aridity (and decrease in the size of the biosphere) across the 41- to 100-ka transition; (2) no change in the Atlantic-Pacific (A-P) δ13C gradient over the last 1.3 Myr, suggesting no change in mean ocean nutrient content accompanied the addition of light carbon; and (3) stronger vertical nutrient fractionation in the North Atlantic in the middle Pleistocene between sites 607 and 552, suggesting weaker North Atlantic Deep Water formation at this time relative to the early and late Pleistocene. We also find evidence for a more pronounced deep recirculation gyre in the western North Atlantic basin in the early Brunhes, as evidenced by “aging” of deep northern basin water (site 607) relative to deep water in the equatorial Atlantic (site 664).

本研究对5条可追溯至1.3百万年前的深海碳同位素δ¹³C记录展开分析，旨在约束41~100千年气候转型期内海洋平均碳同位素组成与温盐环流（thermohaline circulation）的变化。上述数据显示，海洋平均碳化学组成的显著扰动与中更新世气候转型（mid-Pleistocene climate transition）存在密切关联。过去1.3百万年间的显著特征包括：（1）在0.9~1.0百万年前，海洋平均δ¹³C出现约0.3‰的显著下降，随后在距今400千年时恢复至1.0百万年前的初始水平；我们推测该现象源于同位素耗竭的陆地碳一次性输入海洋，这可能与41~100千年转型期内全球干旱程度加剧以及生物圈规模缩小存在关联；（2）过去1.3百万年间，大西洋-太平洋（A-P）δ¹³C梯度未发生显著变化，表明轻碳输入并未伴随海洋平均营养盐含量的改变；（3）中更新世时期，站位607与552所在的北大西洋区域垂直营养盐分馏作用更强，暗示彼时北大西洋深层水形成（North Atlantic Deep Water formation）强度相较于早更新世与晚更新世更弱。本研究还发现布容正向极性时（Brunhes）早期，北大西洋西部盆地存在更为显著的深层再循环环流——这一结论可通过对比赤道大西洋（站位664）的深层水，北部深海盆水（站位607）呈现出的"老化"特征得到佐证。