Sediment geochemistry of ODP Leg 127 sites

The relative effects of paleoceanographic and paleogeographic variations, sediment lithology, and diagenetic processes on the final preserved chemistry of Japan Sea sediments are evaluated by investigating the rare earth element (REE), major element, and trace element concentrations in 59 squeeze-cake whole-round and 27 physical-property sample residues from Sites 794, 795, and 797, cored during ODP Leg 127.
The most important variation in sedimentary chemical composition is the increase in SiO2 concentration through the Pliocene diatomaceous sequences, which dilutes most other major and trace element components by various degrees. This biogenic input is largest at Site 794 (Yamato Basin), moderately developed at Site 797 (Yamato Basin), and of only minor importance at Site 795 (Japan Basin), potentially reflecting basinal contrasts in productivity with the Yamato Basin recording greater biogenic input than the Japan Basin and with the easternmost sequence of Site 794 lying beneath the most productive waters. There are few systematic changes in solid-phase chemistry resulting from the opal-A/opal-CT or opal-CT/quartz silica phase transformations. Most major and trace element concentrations are controlled by the aluminosilicate fraction of the sediment, although the effects of diagenetic silica phases and manganese carbonates are of localized importance.
REE total abundances (Sum REE) in the Japan Sea are strongly dependent upon the paleoceanographic position of a given site with respect to terrigenous and biogenic sources. REE concentrations at Site 794 overall correspond well to aluminosilicate chemical indices and are strongly diluted by SiO2 within the upper Miocene-Pliocene diatomaceous sequence. Eu/Eu* values at Site 794 reach a maximum through the diatomaceous interval as well, most likely suggesting an association of Eu/Eu* with the siliceous component, or reflecting slight incorporation of a detrital feldspar phase. SumREE at Site 795 also is affiliated strongly with aluminosilicate phases and yet is diluted only slightly by siliceous input. At Site 797, SumREE is not as clearly associated with the aluminosilicate fraction, is correlated moderately to siliceous input, and may be sporadically influenced by detrital heavy minerals originating from the nearby rifted continental fragment composing the Yamato Rise.
Ce/Ce* profiles at all three sites increase essentially monotonically with depth and record progressive diagenetic LREE fractionation. The observed Ce/Ce* increases are not responding to changes in the paleoceanographic oxygenation state of the overlying water, as there is no independent evidence to suggest the proper oceanographic conditions. Ce/Ce* correlates slightly better with depth than with age at the two Yamato Basin sites.
The downhole increase in Ce/Ce* at Sites 794 and 797 is a passive response to the diagenetic transfer of LREE (except Ce) from sediment to interstitial water. At Site 795, the overall lack of correlation between Ce/Ce* and Lan/Ybn suggests that other processes mask the diagenetic behavior of all LREEs. First-order calculations of the Ce budget in Japan Sea waters and sediment indicate that ~20% of the excess Ce adsorbed by settling particles is recycled within the water column and that an additional ~38% is recycled at or near the seafloor. Thus, because the remaining excess Ce is only ~10% of the total Ce, there is not a large source of Ce to the deeply buried sediment, further suggesting that the downhole increase in Ce/Ce* is a passive response to diagenetic behavior of the other LREEs. The REE chemistry of Japan Sea sediment therefore predicts successive downhole addition of LREEs to deeply buried interstitial waters.

本研究针对大洋钻探计划（Ocean Drilling Program, ODP）第127航次在794、795和797站位获取的岩心，对其中59件压饼整岩样与27件物性测试样品残渣开展稀土元素（rare earth element, REE）、常量元素及微量元素浓度分析，以此评估古海洋与古地理变化、沉积物岩性以及成岩作用对日本海沉积物最终保存化学组成的相对影响。
沉积物化学组成最显著的变化为上新世硅藻质地层中二氧化硅（SiO₂）浓度持续升高，该过程以不同程度稀释了绝大多数其他常量与微量元素组分。此类生物成因物质输入在794站位（大和盆地）最为显著，797站位（大和盆地）次之，而795站位（日本海盆）仅具微弱影响，这或反映了不同盆地的生产力差异：大和盆地的生物成因输入量高于日本海盆，且794站位最东侧的地层位于最高生产力水体之下。由蛋白石-A（opal-A）/蛋白石-CT（opal-CT）或蛋白石-CT/石英的硅质相转变所引发的固相化学系统性变化极少。多数常量与微量元素浓度受沉积物铝硅酸盐组分控制，尽管成岩硅质相及碳酸锰的影响仅具局域性意义。
日本海沉积物的稀土元素总丰度（ΣREE）强烈依赖于站位相对于陆源与生物成因物质来源的古海洋位置。794站位的稀土元素浓度整体与铝硅酸盐化学指标吻合度较高，且在上中新统-上新统硅藻质地层中被二氧化硅显著稀释。794站位的Eu/Eu*比值在硅藻质地层中同样达到峰值，这大概率表明Eu/Eu*与硅质组分相关，或反映了少量碎屑长石相的混入。795站位的ΣREE也与铝硅酸盐相紧密相关，但仅被硅质输入轻微稀释。而797站位的ΣREE与铝硅酸盐组分的关联度较低，与硅质输入呈中等程度相关，且可能偶尔受到源自附近构成大和隆起的裂谷大陆碎块的碎屑重矿物影响。
三个站位的Ce/Ce*剖面均随深度基本呈单调递增，记录了渐进式的成岩轻稀土元素（light rare earth element, LREE）分馏现象。观测到的Ce/Ce*升高并未响应上覆水体的古海洋氧化状态变化，因为尚无独立证据表明存在此类适宜的海洋环境条件。在两个大和盆地站位中，Ce/Ce*与深度的相关性略优于与沉积年龄的相关性。
794与797站位的Ce/Ce*随钻孔深度升高，是轻稀土元素（除铈外）从沉积物向孔隙水发生成岩迁移的被动响应。795站位的Ce/Ce*与Lan/Ybn比值整体无相关性，表明其他过程掩盖了所有轻稀土元素的成岩行为。针对日本海水体与沉积物中铈预算的一级近似计算显示：沉降颗粒吸附的过剩铈中，约20%在水层内循环，另有约38%在海底或其附近循环。剩余的过剩铈仅占总铈的约10%，因此深埋沉积物中铈的补给量并不大，这进一步表明Ce/Ce*随钻孔深度升高是其他轻稀土元素成岩行为的被动响应。由此，日本海沉积物的稀土元素化学特征表明，轻稀土元素会持续向深埋孔隙水中发生钻孔内的迁移补给。