Abstract The Middle Stone Age (MSA) is important to studies of human evolution because it witnessed the origin of modern Homo sapiens and its dispersal out of Africa and across the rest of the Old World. Obtaining accurate ages for these and other events that occurred during this time interval is of critical importance. Ostrich eggshell (OES) is commonly found in MSA sites, and OES calcite can be dated by radiocarbon and U-series geochronologic techniques. It is important to understand how both techniques are potentially compromised when the eggshell’s pore clusters and intercrystalline microporosity are infilled with younger calcite and/or soil-derived detrital materials. We investigated this question by using high-resolution X-ray computed tomography (HRXCT) to reveal the extent and three-dimensional structure of the pore clusters, and element mapping to evaluate concentrations of detrital elements in modern versus ancient OES within the pore clusters and the crystal (outer) and cone (inner) layers relative to the palisade (central) layer. HRXCT results support earlier findings that OES pores are generally composed of one to several openings on the cone surface that develop into a complex three-dimensional cluster of anastomosing and branching pores that extend through the palisade layer to exit on the crystal surface. These hollow pore clusters provide a considerable total volume for the potential ingress of detrital materials. Element mapping revealed high concentrations of detrital mineral indicator elements (Al, Mg) in ancient relative to modern OES, and in the cone and crystal layers relative to the palisade layer. Within the palisade layer of ancient OES, there are relatively high detrital mineral indicator element concentrations within filled pore clusters. Extracted pore cluster infill from the palisade layer yields significantly higher and more variable concentrations of 232Th (2,200-10,000 ppt) and 238U (35–54 ppb) relative to the non-porous palisade layer (150–3,100 ppt 232Th, and 19–25 ppb 238U). Based on these results, we developed a mechanical preparation method that combines the removal of the pore cluster infill with the more common removal of the cone and crystal layers. Using this method, U–Th analyses of four OES from an MSA archaeological site in NW Ethiopia yielded a mean age of 75.7±4.7 ka (2 SD). The inclusion of the pore cluster infill with the palisade in a different OES from the same site produced a much younger age of 53±1.2 ka (1 SE). Splits of several of these same OES samples prepared by this new method yield AMS 14C ages that are beyond the range of 14C dating (∼50 ka), while ∼60% of the OES analyses of the palisade layer that retained the pore cluster infill yielded much younger ages (27–38 ka). Our results demonstrate that dating OES that retains the pore cluster infill shifts both U–Th and 14C analyses to younger ages. In order to obtain more precise and accurate ages, we recommend that preparation of OES for both U-series and 14C dating include the mechanical removal of both the pore cluster infill and the cone and crystal layers in order to more fully account for the incorporation of Th, U, and C from detrital and authigenic minerals within the OES structure.

中文翻译：

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使用基于微观结构和地球化学的样品制备方法提高鸵鸟蛋壳 U 系列和放射性碳测年的准确性：一项来自埃塞俄比亚西北部中石器时代的研究

摘要 中石器时代 (MSA) 对人类进化研究很重要，因为它见证了现代智人的起源及其从非洲和旧世界其他地区的传播。获得在此时间间隔内发生的这些事件和其他事件的准确年龄至关重要。鸵鸟蛋壳 (OES) 常见于 MSA 站点，OES 方解石可以通过放射性碳和 U 系列年代学技术确定年代。当蛋壳的孔隙簇和晶间微孔被较年轻的方解石和/或土壤衍生的碎屑材料填充时，了解这两种技术如何潜在地受到损害是很重要的。我们通过使用高分辨率 X 射线计算机断层扫描 (HRXCT) 来研究这个问题，以揭示孔隙簇的范围和三维结构，和元素映射，以评估现代与古代 OES 中孔隙簇和晶体（外）层和锥（内）层相对于栅栏（中央）层的碎屑元素浓度。HRXCT 结果支持早期的发现，即 OES 孔通常由锥形表面上的一到几个开口组成，这些开口发展成复杂的三维吻合和分支孔簇，这些孔穿过栅栏层延伸到晶体表面。这些空心孔簇为碎屑材料的潜在进入提供了相当大的总体积。元素映射揭示了相对于现代 OES 的古代以及相对于栅栏层的锥体和晶体层中的高浓度碎屑矿物指示元素（Al、Mg）。在古代 OES 的栅栏层内，在填充的孔隙簇内有相对较高的碎屑矿物指示元素浓度。相对于无孔栅栏层（150–3,100 ppt 232Th 和 19–54 ppb），从栅栏层提取的孔隙簇填充物产生显着更高和更多变化的 232Th（2,200-10,000 ppt）和 238U（35-54 ppb）浓度ppb 238U）。基于这些结果，我们开发了一种机械制备方法，将孔簇填充物的去除与更常见的锥体和晶体层的去除相结合。使用这种方法，来自埃塞俄比亚西北部 MSA 考古遗址的四个 OES 的 U-Th 分析得出平均年龄为 75.7±4.7 ka (2 SD)。在来自同一地点的不同 OES 中包含填充有栅栏的孔隙簇产生了 53±1.2 ka (1 SE) 的年轻得多的年龄。通过这种新方法制备的几个相同 OES 样品的分裂产生了 AMS 14C 年龄超出了 14C 测年范围（~50 ka），而保留孔隙簇填充物的栅栏层的~60% 的 OES 分析产生了更年轻的年龄（27-38 ka）。我们的结果表明，保留孔隙簇填充物的 OES 测年将 U-Th 和 14C 分析转移到更年轻的年龄。为了获得更精确和准确的年龄，我们建议为 U 系列和 14C 测年准备 OES 包括机械去除孔隙簇填充物和锥体和晶体层，以便更充分地考虑纳入来自 OES 结构内碎屑和自生矿物的 Th、U 和 C。而保留孔隙簇填充物的栅栏层的约 60% 的 OES 分析产生了更年轻的年龄（27-38 ka）。我们的结果表明，保留孔隙簇填充物的 OES 测年将 U-Th 和 14C 分析转移到更年轻的年龄。为了获得更精确和准确的年龄，我们建议为 U 系列和 14C 测年准备 OES 包括机械去除孔隙簇填充物和锥体和晶体层，以便更充分地考虑纳入来自 OES 结构内碎屑和自生矿物的 Th、U 和 C。而保留孔隙簇填充物的栅栏层的约 60% 的 OES 分析产生了更年轻的年龄（27-38 ka）。我们的结果表明，保留孔隙簇填充物的 OES 测年将 U-Th 和 14C 分析转移到更年轻的年龄。为了获得更精确和准确的年龄，我们建议为 U 系列和 14C 测年准备 OES 包括机械去除孔隙簇填充物和锥体和晶体层，以便更充分地考虑纳入来自 OES 结构内碎屑和自生矿物的 Th、U 和 C。