Table3_The Effect of Differential Weathering on The Magnetic Properties of Paleosols: A Case Study of Magnetic Enhancement vs. Magnetic Depletion in the Pleistocene Blackwater Draw Formation, Texas.XLSX

The type-section of the Blackwater Draw Formation (BDF) consists of a series of five paleosol horizons developed on eolian deposits and an overlying surficial soil. Previous work has shown that magnetic properties (e.g., χ, ARM, and IRM) as a function of depth in this type-section, display both magnetically enhanced and magnetically depleted signals for different paleosols. To better understand the magnetic mineralogy responsible for these varying responses, various rock-magnetic experiments, scanning electron microscopy, and Mössbauer spectroscopy were conducted on representative samples from the six soil units which constitute the BDF type-section. Our results show that sub-micron hematite [with a minor contribution from single-domain sized hematite (Hc = ∼500 mT) dominates all the soils in terms of weight percent concentration. Whereas, low coercivity (Hc = ∼35 mT or less) magnetite/maghemitized-magnetite grains, largely in the PSD state (Mr/Ms=∼0.14 +/– 0.03588, Hcr/Hc=∼2.68 +/– 0.298789), dominate the magnetic signal. Magnetically depleted soils show a relatively higher proportion of goethite, while magnetically enhanced soils show an increased contribution from SP/SSD magnetite/maghemite phases.By combining our data-set with geochemically-derived climofunctions, we have correlated the magnetically preserved, depleted, and enhanced sections of the type-section to three distinct environmental phases (I-III). The basal sediments of Phase I displays relatively homogenous (neither enhanced nor depleted) magnetic properties due to relatively arid conditions and minimal alteration of southerly derive eolian sands. Conversely, Phase II-III represents a change in weathering intensities and provenance, resulting in a mix of southerly derived sands and northerly derived silts. Phase II, experienced greater precipitation levels, resulting in the dissolution of Fe-oxide phases and thus magnetic depletion. The uppermost Phase III experienced intermediate precipitation intensities resulting in magnetic enhancement.Using previously published age models we tentatively interpret these changing environmental conditions to be influenced by the Middle-Pleistocene Transition (1.2-0.7 Ma), where the Earth’s climatic cycles shifted from a ∼41 kyr to ∼100 kyr cycles. However, ambiguities persist due to uncertainties in the currently published age model. Due to the complexity of the magnetic signal, we recommend future studies utilize a holistic approach, incorporating rock-magnetic, geochemical, and microscopy observations for more accurate reconstruction of regional paleoenvironments.

黑水槽地层的典型剖面区由一系列五个在风成沉积物上发育的古土壤层及覆盖其上的表层土壤组成。前期研究表明，该典型剖面区中磁性质（例如，χ、ARM 和 IRM）随深度的变化表现出不同古土壤的磁增强和磁耗减信号。为更好地理解导致这些不同反应的磁性矿物学，我们对构成黑水槽地层典型剖面区的六个土壤单元的代表样品进行了多种岩石磁学实验、扫描电子显微镜和 Mössbauer 光谱分析。我们的结果表明，亚微米级的赤铁矿（包括来自单畴赤铁矿（Hc ≈ 500 mT）的微小贡献）在所有土壤中以重量百分比浓度占主导地位。而低矫顽力（Hc ≈ 35 mT 或更低）的磁铁矿/磁化磁铁矿颗粒，大部分处于 PSD 状态（Mr/Ms ≈ 0.14 ± 0.03588，Hcr/Hc ≈ 2.68 ± 0.298789），在磁性信号中占主导地位。磁耗减土壤表现出相对较高的针铁矿比例，而磁增强土壤则显示出 SP/SSD 磁铁矿/磁化磁铁矿相的贡献增加。通过将我们的数据集与地质化学衍生出的气候函数相结合，我们将典型剖面区的磁保留、耗减和增强部分与三个不同的环境阶段（I-III）相对应。第一阶段的基础沉积物显示出相对均一（既未增强也未耗减）的磁性特性，这是由于相对干旱的气候条件和南方来源的风成沙的轻微变化。相反，第二阶段至第三阶段代表了风化强度和来源的变化，导致南方来源的沙和北方来源的粉砂的混合。第二阶段经历了更高的降水量，导致 Fe-氧化物相的溶解，从而引起磁性耗减。最上层的第三阶段经历了中等降水强度，导致磁性增强。利用先前发表的年龄模型，我们暂且将这些变化的环境条件解释为中中新世过渡期（1.2-0.7 Ma）的影响，在这一时期，地球的气候周期从约 41 kyr 转变为约 100 kyr。然而，由于目前发表的年龄模型的不确定性，仍存在模糊性。由于磁性信号的复杂性，我们建议未来的研究采用整体方法，结合岩石磁学、地球化学和显微镜观察，以更精确地重建区域古环境。