Data from: Dating basal peat: The geochronology of peat initiation revisited

Attributing the start of peat growth to an absolute timescale requires dating the bottom of peat deposits overlying mineral sediment, often called the <em>basal peat</em>. Peat initiation is reflected in the stratigraphy as a gradual transition from mineral sediment to increasingly organic material, up to where it is called peat. So far, varying criteria have been used to define basal peat, resulting in divergent approaches to date peat initiation. The lack of a universally applicable and quantitative definition, combined with multiple concerns that have been raised previously regarding the radiocarbon dating of peat, may result in apparent ages that are either too old or too young for the timing of peat initiation. Here, we aim to formulate updated recommendations for dating peat initiation. We provide a conceptual framework that supports the use of the organic matter (OM) gradient for a quantitative and reproducible definition of the mineral-to-peat transition (i.e., the stratigraphical range reflecting the timespan of the peat initiation process) and the layer defined as <em>basal peat</em> (i.e., the stratigraphical layer that is defined as the bottom of a peat deposit). Selection of dating samples is often challenging due to poor preservation of plant macrofossils in basal peat, and the representativity of humic and humin dates for the age of basal peat is uncertain. We therefore analyse the mineral-to-peat transition based on three highly detailed sequences of radiocarbon dates, including dates of plant macrofossils and the humic and humin fractions obtained from bulk samples. Our case study peatland in the Netherlands currently harbours a bog vegetation, but biostratigraphical analyses show that during peat initiation the vegetation was mesotrophic. Results show that plant macrofossils provide the most accurate age in the mineral-to-peat transition and are therefore recommendable to use for 14C dating basal peat. If these are unattainable, the humic fraction provides the best alternative and is interpreted as a terminus-ante-quem for peat initiation. The potential large age difference between dates of plant macrofossils and humic or humin dates (up to ~1700 years between macrofossil and humic ages, and with even larger differences for humins) suggests that studies reusing existing bulk dates of basal peat should take great care in data interpretation. The potentially long timespan of the peat initiation process (with medians of ~1000, ~1300 and ~1500 years within our case study peatland) demonstrates that choices regarding sampling size and resolution need to be well substantiated. We summarise our findings as a set of recommendations for dating basal peats, and advocate the widespread use of OM determination to obtain a low-cost, quantitative and reproducible definition of <em>basal peat</em> that eases intercomparison of studies.

若要将泥炭生长的起始时间归因于绝对年代标尺，则需对覆于矿物沉积物之上的泥炭沉积底部——通常称为**基底泥炭（basal peat）**——开展定年工作。泥炭起始过程在地层记录中表现为从矿物沉积物到有机质含量逐渐升高的过渡序列，最终过渡为纯泥炭层。迄今为止，学界已采用多种不同标准定义基底泥炭，这导致泥炭起始定年的方法存在显著分歧。由于缺乏普适且定量化的定义，加之此前学界针对泥炭放射性碳定年提出的多项顾虑，可能会导致泥炭起始时间的表观年龄出现偏老或偏年轻的偏差。本研究旨在提出泥炭起始定年的更新版建议方案。我们构建了一套概念框架，可依托有机质（organic matter, OM）梯度实现矿物-泥炭过渡带（即反映泥炭起始过程时间跨度的地层范围）与**基底泥炭（basal peat）**层位（即被界定为泥炭沉积底部的地层单元）的定量化、可复现定义。由于基底泥炭中植物大化石的保存状况较差，定年样品的选取往往颇具挑战；同时，腐殖质与腐黑物组分的定年结果对基底泥炭年代的代表性尚不明确。据此，我们基于三套高精度放射性碳定年序列开展矿物-泥炭过渡带分析，分析数据涵盖植物大化石定年结果，以及从bulk样品（bulk samples）中提取的腐殖质与腐黑物组分的定年数据。本研究的案例泥炭地位于荷兰，当前发育有沼泽植被，但生物地层学分析显示，在泥炭起始阶段，该区域的植被为中营养型（mesotrophic）。研究结果表明，植物大化石可提供矿物-泥炭过渡带中最为精准的年代数据，因此推荐将其用于基底泥炭的¹⁴C定年。若无法获取植物大化石样品，腐殖质组分则是最佳替代方案，可将其解释为泥炭起始时间的前限年代（terminus ante quem）。植物大化石定年结果与腐殖质或腐黑物定年结果之间可能存在显著年龄差（大化石与腐殖质年龄的差值可达约1700年，腐黑物的差值甚至更大），这表明复用现有基底泥炭bulk样品定年数据的研究需在数据解译过程中格外谨慎。泥炭起始过程的时间跨度可能较长（本研究案例泥炭地内的中位值分别约为1000年、1300年和1500年），这说明采样规模与分辨率的选择需要有充分的依据支撑。我们将研究结果总结为一套基底泥炭定年的建议方案，并倡导广泛采用有机质测定方法，以获取低成本、定量化且可复现的**基底泥炭（basal peat）**定义，从而简化不同研究之间的对比工作。