Iron oxide minerals are widely present in soil and known to be closely associated with soil-forming environments. In this study, a detailed quantitative analysis of the iron oxide minerals in the Late Pleistocene hard clay of the Yangtze River Delta (YRD) was conducted using new integrated methods of magnetism, selective chemical-dissolution, and diffuse reflectance spectroscopy. A variety of established chemical weathering indexes and optically stimulated luminescence (OSL) chronology were also applied in the investigations of the hard clay in the study area. These methods allowed for detailed examinations of the chemical weathering, sedimentary environmental conditions, and the post-depositional processes of the hard clay deposits in the YRD. This study’s A-CN-K diagram revealed that the hard clay had entered an intermediate weathering stage. In addition, various chemical weathering indexes demonstrated that the hard clay had undergone three stages of chemical weathering from bottom to top as follows: 1) A relatively high stage of chemical weathering; 2) A low chemical weathering stage with a slightly increasing trend; and 3) A relatively high chemical weathering stage with a weakly decreasing trend. It was found that when χ, χ_fd, Fe_d/Fe_t, Gt, and Hm/Gt were not reliable indicators of chemical weathering for the hard clay, Fe_d could be a useful proxy. In the current study, based on the OSL ages and several paleoclimatic proxies, it was suggested that the climate conditions in the YRD had transitioned from a relatively weak to a strong East Asian summer monsoon phase during the periods between the marine oxygen isotope stages (MIS) 4 and MIS 3. Furthermore, the low values of χ and χ_fd in the hard clay indicated an intense dissolution of fine-grained strong magnetic minerals. Meanwhile, the relatively high Fe_o/Fe_d ratios in the superficial horizon displayed the predominance of poorly crystalline forms of iron oxides. In summary, the analysis results implied that the hard clay had undergone post-burial processes of ferrolysis, as well as dissolved organic matter translocation from the overlying Holocene sediment. The aforementioned post-burial processes for the hard clay had occurred under reduction–oxidation cycles, which were mainly driven by the rise of the groundwater levels with the Holocene transgression.

氧化铁矿物广泛存在于土壤中，并且已知与土壤形成环境密切相关。在这项研究中，使用磁性、选择性化学溶解和漫反射光谱的新集成方法对长江三角洲 (YRD) 晚更新世硬质粘土中的氧化铁矿物进行了详细的定量分析。各种已建立的化学风化指数和光激发光 (OSL) 年代学也被应用于研究区硬粘土的调查。这些方法允许详细检查长三角硬粘土矿床的化学风化、沉积环境条件和沉积后过程。本研究的 A-CN-K 图显示硬粘土已进入中间风化阶段。此外，各种化学风化指标表明，硬质粘土自下而上经历了三个化学风化阶段：1）较高的化学风化阶段；2）低化学风化阶段，略有增加趋势；3）化学风化阶段较高，呈弱下降趋势。发现当 χ, χ_fd、Fe _d /Fe _t、Gt 和 Hm/Gt 不是硬粘土化学风化的可靠指标，Fe _d可能是一个有用的替代指标。在目前的研究中，基于 OSL 年龄和几个古气候代理，表明在海洋氧同位素阶段 (MIS ) 4 和 MIS 3. 此外，硬质粘土中 χ 和 χ _fd的低值表明细粒强磁性矿物的强烈溶解。同时，相对较高的 Fe _o /Fe _d表层中的比例显示出铁氧化物的低结晶形式占优势。总之，分析结果表明硬粘土经历了铁解的后埋藏过程，以及上覆全新世沉积物的溶解有机质易位。上述硬粘土的后埋藏过程发生在还原-氧化循环下，这主要是由全新世海侵引起的地下水位上升驱动的。