Lignin oxidation products in soil, dripwater and speleothems from four different sites in New Zealand

Inken Heidke,Thorsten Hoffmann,Andrew Pearson,John Hellstrom,Sebastian F. M. Breitenbach,Denis Scholz,Adam Hartland

doi:10.5194/bg-18-2289-2021

Abstract

Abstract. Lignin oxidation products (LOPs) are widely used as vegetation proxies in climate archives, such as sediment and peat cores. The total LOP concentration, Σ8, provides information on the abundance of vegetation, while the ratios C/V and S/V of the different LOP groups also provide information on the type of vegetation. Recently, LOP analysis has been successfully applied to speleothem archives. However, there are many open questions concerning the transport and microbial degradation of LOPs on their way from the soil into the cave system. These processes could potentially alter the original source-dependent LOP signals, in particular the C/V and S/V ratios, and thus complicate their interpretation in terms of past vegetation changes. We analyzed LOPs in leaf litter and different soil horizons as well as dripwater and flowstone samples from four different cave sites from different vegetation zones in New Zealand using ultrahigh-performance liquid chromatography coupled to high-resolution mass spectrometry. We test whether the original source-dependent LOP signal of the overlying vegetation is preserved and can be recovered from flowstone samples and investigate how the signal is altered by the transport from the soil to the cave. The LOP concentrations range from mg g−1 in the soil to ng g−1 in the flowstones. Our results demonstrate that, from the soil to the flowstone, the C/V and S/V ratios both increase, while the total lignin content, Σ8, strongly decreases. This shows that the LOP signal is strongly influenced by both transport and degradation processes. Nevertheless, the relative LOP signal from the overlying soil at the different cave sites is preserved in the flowstone. We emphasize that for the interpretation of C/V and S/V ratios in terms of past vegetation changes, it is important to compare only samples of the same type (e.g., speleothem, dripwater or soil) and to evaluate only relative variations.

Highlights

Climate archives provide the means to study the climate and environment of the past, which is necessary to put ongoing climate change into a historic framework
From the soil to the flowstone, the C/V and S/V ratios both increase, while the total lignin content, 8, strongly decreases
The relative Lignin oxidation products (LOPs) signal from the overlying soil at the different cave sites is preserved in the flowstone

Summary

Introduction

Climate archives provide the means to study the climate and environment of the past, which is necessary to put ongoing climate change into a historic framework. Speleothems are valuable climate archives because they are ubiquitous across climatic and vegetational zones, preserve a range of inorganic and organic proxies (Fairchild and Baker, 2012; Blyth et al, 2016), and can be accurately dated up to ca. Fatty acids, originating from both plants and soil microorganisms, have been extracted from speleothems to study soil and vegetation activity (Blyth et al, 2006; Bosle et al, 2014), and long-chain n-alkanes from plant leaf waxes have been used to investigate vegetation changes (Xie, 2003; Blyth et al, 2007, 2011). The use of n-alkane chain length distributions to distinguish between different plant groups has been debated (Bush and McInerney, 2013; Blyth et al, 2016)

Methods

Results

Conclusion