Paleo-Productivity reconstruction based on Ba content in marine sediments

Productivity changes in the tropical-subtropical East-Atlantic are reconstructed using a multiproxy approach. This involves comparing the barium content of the marine surface sediment and sediment core records with other export productivity proxies. Recent biogenic barium accumulation rates (Babio-AR) are shown to generally reflect export productivity (Pexp) in the modern East Atlantic. This relationship may also depend on the planktonic ecology of the surface water, which is dominated by the Ba-rich skeletons (celestite of acantharians) in the open marine environment. Coastal and open marine transfer functions are derived by calibrating Babio-AR with export productivity. However, the sedimentary Ba signal is found to decrease with depth below 1850 m, probably due to increased dissolution. Results also show that Ba preservation is not dependent on the amount of fine fraction (< 2µm). lt remains uncertain whether increased silica dissolution through open marine aeolian Fe-input (Bishop, 1988; Takeda, 1998) favours a decreased Ba-flux. Longterm Pexp-changes determined using the new Ba transfer functions significantly agree with those derived from other methods (i.e. TOC) in open marine locations in the northern margin of the North Atlantic subtropical gyre over the last 40 ka and the last 330 ka in the equatorial divergence zone. Export productivity is shown to be 2-3 times higher than today during cold periods in the equatorial high productivity zone and possibly up to 6 times higher during stages 6 and 8. Productivity cycles are evidently controlled by the lateral advection of nutrient-rich intermediate waters from high southern latitudes.本研究采用多指标法（multiproxy approach）重建了热带-亚热带东大西洋海域的生产力变化。该方法通过对比海洋表层沉积物与沉积物岩芯记录中的钡含量，并结合其他输出生产力（export productivity）代用指标开展分析。 现有研究表明，现代东大西洋海域的生源钡累积速率（biogenic barium accumulation rates, Babio-AR）整体可反映输出生产力（export productivity, P_exp），且该关联可能受表层水体浮游生态调控：在开阔海环境中，钡富集骨骼——棘胞虫（acantharians）的天青石占据主导地位。研究通过将生源钡累积速率与输出生产力进行校准，构建了近岸与开阔海转换函数。但同时发现，沉积物中的钡信号在水深1850米以下呈降低趋势，这可能与溶解作用增强有关。此外，钡的保存状况与细粒组分（粒径<2微米）的含量并无关联。目前仍不明确，通过大气粉尘输入的铁（Bishop, 1988; Takeda, 1998）所引发的硅质溶解增强，是否会抑制钡通量（Ba-flux）的提升。 基于新建钡转换函数得到的长期输出生产力变化序列，在过去4万年北大西洋亚热带环流（North Atlantic subtropical gyre）北缘开阔海区域，以及过去33万年赤道辐散区，与其他代用指标（即总有机碳（Total Organic Carbon, TOC））的重建结果显著吻合。在赤道高生产力区的冷期阶段，输出生产力较现代提升2~3倍；在第6、8冰期阶段，生产力甚至可能达到现代的6倍之多。研究还发现，生产力周期主要受南半球高纬度海域富营养中层水的侧向平流输送调控。