n-alkane carbon isotope ratios in plants and recent lake sediments from Cameroon

Trees and shrubs in tropical Africa use the C3 cycle as a carbon fixation pathway during photosynthesis, while grasses and sedges mostly use the C4 cycle. Leaf-wax lipids from sedimentary archives such as the long-chain n-alkanes (e.g., n-C27 to n-C33) inherit carbon isotope ratios that are representative of the carbon fixation pathway. Therefore, n-alkane d13C values are often used to reconstruct past C3/C4 composition of vegetation, assuming that the relative proportions of C3 and C4 leaf waxes reflect the relative proportions of C3 and C4 plants. We have compared the d13C values of n-alkanes from modern C3 and C4 plants with previously published values from recent lake sediments and provide a framework for estimating the fractional contribution (areal-based) of C3 vegetation cover (fC3) represented by these sedimentary archives. Samples were collected in Cameroon, across a latitudinal transect that accommodates a wide range of climate zones and vegetation types, as reflected in the progressive northward replacement of C3-dominated rain forest by C4-dominated savanna. The C3 plants analysed were characterised by substantially higher abundances of n-C29 alkanes and by substantially lower abundances of n-C33 alkanes than the C4 plants. Furthermore, the sedimentary d13C values of n-C29 and n-C31 alkanes from recent lake sediments in Cameroon (-37.4 per mil to -26.5 per mil) were generally within the range of d13C values for C3 plants, even when from sites where C4 plants dominated the catchment vegetation. In such cases simple linear mixing models fail to accurately reconstruct the relative proportions of C3 and C4 vegetation cover when using the d13C values of sedimentary n-alkanes, overestimating the proportion of C3 vegetation, likely as a consequence of the differences in plant wax production, preservation, transport, and/or deposition between C3 and C4 plants. We therefore tested a set of non-linear binary mixing models using d13C values from both C3 and C4 vegetation as end-members. The non-linear models included a sigmoid function (sine-squared) that describes small variations in the fC3 values as the minimum and maximum d13C values are approached, and a hyperbolic function that takes into account the differences between C3 and C4 plants discussed above. Model fitting and the estimation of uncertainties were completed using the Monte Carlo algorithm and can be improved by future data addition. Models that provided the best fit with the observed d13C values of sedimentary n-alkanes were either hyperbolic functions or a combination of hyperbolic and sine-squared functions. Such non-linear models may be used to convert d13C measurements on sedimentary n-alkanes directly into reconstructions of C3 vegetation cover.

热带非洲的乔木与灌木在光合作用过程中以C3循环（C3 cycle）作为固碳途径，而草本植物和莎草大多采用C4循环（C4 cycle）。来自长链正构烷烃（long-chain n-alkanes，例如n-C27至n-C33）这类沉积记录（sedimentary archives）的叶蜡脂类，其碳同位素比值（carbon isotope ratios）可反映对应植物的固碳途径。因此，正构烷烃的δ¹³C值（d13C values）常被用于重建古植被的C3/C4组成，其假设前提为：C3与C4植物叶蜡的相对比例可反映二者在植被中的相对占比。本研究将现代C3、C4植物中正构烷烃的δ¹³C值与已发表的近代湖泊沉积物数据进行对比，构建了一套可估算上述沉积记录所代表的C3植被覆盖分数贡献量（基于面积的areal-based，记为fC3）的分析框架。研究样本采集自喀麦隆（Cameroon）境内一条纬度断面（latitudinal transect），该断面覆盖多样气候带与植被类型，呈现出以C3为主的雨林向北逐渐被C4为主的稀树草原（savanna）取代的过渡特征。相较于C4植物，所分析的C3植物表现出n-C29正构烷烃丰度显著更高、n-C33正构烷烃丰度显著更低的特征。此外，喀麦隆近代湖泊沉积物中n-C29与n-C31正构烷烃的δ¹³C值范围为-37.4‰至-26.5‰，整体处于C3植物的δ¹³C值区间内——即便在流域植被以C4植物为主的采样点亦是如此。在此类情形下，若利用沉积正构烷烃的δ¹³C值重建C3/C4植被占比，简单线性混合模型（linear mixing models）无法得到准确结果，且会高估C3植被的占比，该偏差可能源于C3与C4植物在叶蜡生成、保存、搬运及沉积过程中的差异。因此，本研究以C3、C4植物的δ¹³C值作为端元（end-members），测试了一组非线性二元混合模型（non-linear binary mixing models）。这些非线性模型包含一种S型函数（正弦平方函数，sine-squared），可描述当δ¹³C值趋近上下限时fC3值的小幅波动，以及一种考虑了前述C3与C4植物差异的双曲函数（hyperbolic function）。模型拟合与不确定性估算通过蒙特卡洛算法（Monte Carlo algorithm）完成，未来可通过补充数据进一步优化模型精度。与沉积正构烷烃实测δ¹³C值拟合效果最佳的模型为双曲函数，或是双曲函数与正弦平方函数的组合形式。这类非线性模型可直接将沉积正构烷烃的δ¹³C测量值转换为C3植被覆盖度的重建结果。