Denudation and weathering rates of carbonate landscapes from meteoric 10Be/9Be ratios

We provide sample information and geochemical data for obtaining erosion, weathering, and denudation rates from a framework based cosmogenic meteoric 10Be versus stable 9Be (10Be/9Be) ratios. We modified this published silicate framework (von Blanckenburg et al., 2012) to carbonate landscapes, and performed thorough ground-truthing and testing of assumptions, as this is the first application of the framework for carbonate lithologies. The most important methodological findings are as follows:

1) We amended a sequential extraction step specific for solubilizing total carbonate-bound Be using acetic acid. As this extraction cannot distinguish between secondary and primary carbonate, we employed carbon stable isotopes to obtain the fraction of Be associated with secondary carbonate. We find that >90% of total carbonate-bound Be is bound to secondary carbonate, meaning that distinguishing between secondary and primary carbonate and employing carbon stable isotopes may not be necessary.

2) Using radiogenic strontium isotope ratios we found that about a third of the 9Be contained in secondary carbonate is derived from the dissolution of silicate phases, likely clastic impurities such as clays. These silicate phases also adsorb meteoric 10Be during weathering. The method is thus applicable to pure limestone as well as mixed carbonate-siliciclastic lithologies.

3) Total 9Be concentrations in bedrock are heterogeneous in the Jura, and are potentially controlled by the amount of silicate impurities contained in limestone. Yet the average 9Beparent in summed carbonate- and silicate-bound fractions (0.07 ug/g) is about 9 times lower than values from existing rock databases. In limestones studies, 9Beparent must be thus determined case-by-case on local bedrock.

4) The analysis of partition coefficients Kd for 10Be and 9Be, respectively, and very similar 10Be/9Be ratios show that dissolved Be has equilibrated between reactive (amorphous and crystalline Fe-oxides) and secondary carbonate phases. Secondary carbonate phases are thus part of the reactive Be pool in limestone settings.

5) As in previous studies in silicate lithologies 10Be and 9Be concentrations show pronounced differences between soil and sediment samples that we attribute to grain size dependence and sorting. The 10Be/9Be ratios however cover a remarkably narrow range for all samples, resulting a in narrow range in denudation rates.

6) The fraction of 9Be released by weathering and partitioned into the secondary reactive or dissolved phase serves as a Be-specific proxy for the degree of weathering.

7) The atmospheric depositional flux of 10Be estimated for the Jura mountains from concentrations of dissolved and particulate 10Be and river gauging is about 80% of estimates from independent global GCM-based distribution maps. The GCM estimates thus provide sufficient accuracy.

From application of these new principles, weathering and erosion in the French Jura Mountains can be described as follows: The proportion of weathering in total denudation W/D is >0.9, due to the high purity of the limestone that almost completely dissolved except for a small silicate mineral fraction that, however, carries 50% of the bedrock’s 9Be. Resulting 10Be/9Be-derived denudation rates are on average 300 t/km2/yr for soils and 580 t/km2/yr for river sediments. The soil-derived values agree well with previous estimates from gauging data despite their entirely different (decadal vs. millennial) integration time scales. That sediment-derived denudation rates exceed those from soil we attribute to a 30-60% contribution from subsurface bedrock weathering. On a global scale, our data provides the first cosmogenic-based denudation rates for the precipitation (MAP) range of 1200 to 1700 mm/yr under a temperate climate and dense vegetation cover. Previous millennial-scale denudation rates from in situ-36Cl in calcite from less vegetated sites do not exceed 250 t/km2/yr in this precipitation range. With 500-600 t/km2/yr our denudation rates peak at MAP of 1200-1300 mm/yr, and then show a trend of decreasing D with increasing MAP.

本数据集提供了基于宇宙成因大气10Be (cosmogenic meteoric 10Be)与稳定9Be (stable 9Be)比值 (10Be/9Be ratios)的框架，用于获取侵蚀、风化及剥蚀速率所需的样品信息与地球化学数据。我们将已发表的硅酸盐岩框架 (silicate framework，von Blanckenburg等，2012) 修改适配至碳酸盐岩景观 (carbonate landscapes)，并对相关假设开展了全面的实地验证与检验——这是该框架首次应用于碳酸盐岩岩性。本研究的核心方法学发现如下：

1） 我们优化了针对乙酸 (acetic acid) 溶解全部碳酸盐结合态铍的专属连续萃取法 (sequential extraction) 步骤。由于该萃取法无法区分次生与原生碳酸盐，我们借助碳稳定同位素 (carbon stable isotopes) 来识别与次生碳酸盐结合的铍占比。研究发现，超过90%的碳酸盐结合态总铍均结合于次生碳酸盐中，这表明区分次生与原生碳酸盐并使用碳稳定同位素可能并非必要步骤。

2） 借助放射成因锶同位素比值 (radiogenic strontium isotope ratios) 分析，我们发现次生碳酸盐中约三分之一的9Be来自硅酸盐相 (silicate phases) 的溶解，此类硅酸盐相大概率为黏土 (clays) 等碎屑杂质 (clastic impurities)。此类硅酸盐相在风化过程中还会吸附大气成因10Be。因此，本方法既适用于纯石灰岩 (limestone)，也可应用于碳酸盐-碎屑岩混合岩性 (carbonate-siliciclastic lithologies)。

3） 汝拉山脉 (Jura) 的基岩 (bedrock) 总9Be浓度存在非均质性，其潜在控制因素为石灰岩中所含的硅酸盐杂质总量。不过，碳酸盐结合相与硅酸盐结合相的总9Be平均值（0.07 μg/g）约为现有岩石数据库中数值的1/9。因此，在石灰岩相关研究中，必须针对当地基岩逐个测定母岩9Be (9Beparent) 含量。

4） 分别针对10Be与9Be的分配系数 (partition coefficients，简称Kd) 分析结果，以及二者极为相近的10Be/9Be比值均表明，溶解态铍在活性相（非晶质与晶质铁氧化物 (Fe-oxides)）与次生碳酸盐相之间达到了平衡。因此，次生碳酸盐相是石灰岩环境中活性铍库的组成部分。

5） 与此前硅酸盐岩岩性的研究结果一致，土壤 (soil) 与沉积物样品 (sediment samples) 中的10Be与9Be浓度存在显著差异，我们将其归因于粒度依赖性 (grain size dependence) 与分选作用 (sorting)。不过，所有样品的10Be/9Be比值均处于极窄的区间内，进而得到了范围狭窄的剥蚀速率 (denudation rates) 结果。

6） 经风化释放并分配至次生活性相或溶解相的9Be占比，可作为表征风化程度的铍专属代用指标。

7） 基于溶解态与颗粒态10Be浓度以及河流流量监测 (river gauging) 数据，我们估算得到汝拉山脉的10Be大气沉积通量 (atmospheric depositional flux) 约为基于独立全球气候模式 (Global Climate Model，简称GCM) 分布图估算值的80%。因此，GCM估算结果具备足够的精度。

基于上述新方法的应用，法国汝拉山脉的风化 (weathering) 与侵蚀 (erosion) 特征可描述如下：由于石灰岩纯度极高，几乎完全被溶解，仅残留少量硅酸盐矿物组分——但该组分携带了基岩50%的9Be，因此总剥蚀中的风化占比 (total denudation W/D，简称W/D) 大于0.9。基于10Be/9Be比值计算得到的剥蚀速率平均值为：土壤样品300 t/km²/yr，河流沉积物样品580 t/km²/yr。尽管二者的积分时间尺度完全不同（数十年vs. 数千年），土壤样品得到的估算值与此前基于流量监测数据 (gauging data) 得到的结果吻合良好。我们认为沉积物衍生剥蚀速率高于土壤样品结果的原因，是地下基岩风化贡献了30%~60%的剥蚀量。在全球尺度上，本数据集首次提供了温带气候与茂密植被覆盖下，年平均降水量 (Mean Annual Precipitation，简称MAP) 介于1200~1700 mm区间内的宇宙成因核素剥蚀速率数据。此前基于植被覆盖度较低区域方解石 (calcite) 中原位36Cl (in situ-36Cl) 得到的千年尺度剥蚀速率，在该降水量区间内均未超过250 t/km²/yr。本研究得到的剥蚀速率峰值为500~600 t/km²/yr，对应MAP为1200~1300 mm/yr，随后剥蚀速率D随MAP升高呈现下降趋势。

搜集汇总

背景与挑战

背景概述

该数据集提供了基于宇宙成因10Be/9Be比率的碳酸盐景观风化和剥蚀速率研究数据，首次将该方法应用于碳酸盐岩性并进行了方法学改进。数据集包含法国侏罗山脉的样本信息和地球化学数据，揭示了该地区风化和剥蚀过程的特点及速率。