Human and climate global-scale imprint on sediment transfer during the Holocene

• global soil erosion
• lake records
• ¹⁴C ages
• pollens
• varved sediments

Changes in human land use have increased soil erosion rates globally (1, 2) and have led to substantial alterations in soil stability and productivity, functioning of river ecosystems, and lateral transfers of sediments and carbon, affecting ultimately long-term change of climate (3⇓–5). The impacts of human land uses and climate on soil erosion amplitude and spatial extent are closely tied to the timescale considered. On geological timescales of millions of years, erosion is controlled by climate and tectonic cycles (6), which, in turn, regulates atmospheric CO₂ and climate through chemical weathering (7) and organic C burial in sediments (8). During the course of the Holocene, anthropogenic modifications of catchments, including vegetation clearance and burning, as well as agricultural and urban expansion, have led to rapid fluctuations in local to regional soil erosion rates (9⇓–11), and ultimately this has accelerated erosion 10- to 100-fold in some regions (1). However, it is still unclear to what extent climate and human factors have interacted and controlled long-term (i.e., >100 y) soil erosion rates, and when a significant portion of the Earth surface shifted to human-driven soil erosion rates (3).

Natural archives preserved as lake sediments provide a key source of evidence for assessing soil erosion that occurs in lake catchments and is integrative of all fluxes and processes that remove soil, rock, or dissolved material from the watershed, including gully, till, or rill erosion. For many lakes, sediment accumulation rates (SARs) from cores collected in profundal depositional zones can provide an estimation of gross changes in the rate of deposition of allochthonous sediment related to erosion (12), although SARs variability can also be related to other processes (Methods). Due to their relatively high temporal resolution and often continuous history of deposition, lake cores are valuable archives of catchment-scale soil erosion dynamics, (i.e., temporal variability in catchment erosion intensity). Furthermore, the abundance of Holocene lake records and the diversity of catchment systems around the globe have established lake sediment cores as strategic data repositories for a better understanding of transfer of matter on the surface of Earth.

The objectives of the present study are to identify first increases in sediment supplies to lakes to infer soil erosion temporal dynamics across globally distributed sites to quantify spatiotemporal patterns and to investigate the effects of climate and land cover change (LCC) during the Holocene using SAR records from lakes. Syntheses such as this one have the potential to make key advances in our understanding of critical events for Earth system processes. A major limitation for identifying the transition from an overarchingly climatically controlled system to one in which human drivers explain changes in soil erosion rates is the lack of spatio-temporally resolved empirical data, which are needed to discriminate between local stressors mediated primarily by human activities and climatic factors (13, 14). Furthermore, our gap in knowledge about changes in global soil erosion rates limits our ability to clearly define of the onset of the Anthropocene (15). Finally, the lack of synthesis also limits our understanding of the timing and extent of increased soil erosion and hinders predictions about long-term total carbon losses from soils and land-atmosphere exchanges (3).

In this study, we bring together a global synthesis of pollen samples (n = 43,669) and ¹⁴C dates (n = 3,980) to estimate temporal trends in land cover and lake SARs in 632 lake-watersheds (Fig. 1 and Dataset S1). The ages of the first increase in soil erosion in lake-watersheds were then derived from the SAR trends (Methods). The lake pollen records (retrieved from three pollen databases [European (EPD), Global (GPD), and African Pollen Database (APD)]) were grouped into 5 plant-functional types and 2 land-cover types (arboreal and nonarboreal). Our a priori prediction is that decreases in the proportion of arboreal pollen (AP) would reflect land cover changes (i.e., land clearance) that, in turn, would lead to soil degradation and erosion. Trends in long-term precipitation, air temperature, and wind were assessed using MPI-ESM-1.2LR transient Holocene simulations (Dataset S1). Generalized additive models (GAMs) were used to estimate the long-term contribution of land cover and climate on erosion. Our approach is summarized in SI Appendix, Figs. S1 and S2.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 1.

(A) Location of the 632 study sites and changes in lake sediment rates over the last 10,000 y. Increases in lake sediment rates (red dots) were observed in 35.1% of the watersheds according to M-K τ coefficients; the rest of the sites show a decrease (green dots) or steady (white dots) rates. (B) Distribution of M-K trend τ coefficients. (C) Number of dated cores containing a pollen record and distribution of ¹⁴C dates (intervals = 12,000 y) used to calculate SARs.

Sampled lake-catchments capture a comparable range of morphometric properties (i.e., site elevation, watershed area, lake area, and lake depth) relative to the 14.3 million lakes in the global HYDROLAKE database (16), as well as a wide range of soil types, land cover, and climatic conditions (SI Appendix, Figs. S1A–S3 and Datasets S2–S5). Based on our analyses and reconstruction of SARs (Methods and SI Appendix, Fig. S4), we found that ∼35% of the long-term records of SARs for 632 sites were essentially increasing (Kendall’s tau coefficient τ > 0.2, P value <0.05), while 26% were decreasing (τ < −0.2, P value <0.05) and 39% were stationary or showed no significant trends (Fig. 1 B and C and Table 1). Piecewise regression modeling on each time series (Methods) showed that the global signal of SAR and mass accumulation rates (i.e., MAR, corrected from compaction, see Methods) increased since 4,000 calendar years before the present (cal. BP) (i.e., mean breakpoint year = 4,150 cal. BP [minimum-maximum range 3,250–4,547]). The fraction of lakes recording positive anomalies in SARs also increase circa 4,000 cal. BP, at rate of 12 ± 0.2% ka⁻¹ (P < 0.01; Fig. 2 A–C).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 2.

Trends in lake SARs (proxy of erosion), land cover as well as climate change during the Holocene. (A and B) Density plots of the anomalies in SARs and AP percentages (43,669 pollen samples), respectively, for 632 ¹⁴C-dated sites (3,980 calibrated ¹⁴C ages). Global trends for SARs are shown in 632 lakes based on an GAM, with 95% confidence intervals on the predicted means (C), as SARs medians trends calculated every 50 y (n = 632 sites) and point breaks detected on the SAR medians curve global trend based on a bootstrap distribution (D). (E) Changes in land cover are shown as trends AP percentage for the subset of sites (n = 116) recording an increase in SARs over time (solid green line), and for a subset of sites (n = 287) recording no significant changes in SARs over time, denoted here as benchmark sites (dashed green line). (F) From ref. 15: trends in global temperature anomalies (relative to the average over the period 1961–1990 in blue) and atmospheric CO₂ (red) from the GRIP ice core59.

A common observation across our sites recording accelerated SARs during the last 4,000 y was the trend of deforestation (10), which coincided with the increase of SARs (Fig. 2 A–E). Our statistical analyses support the conclusion that land cover change was the leading driver for the onset of accelerated lake sediment deposition. Assessment of the relative contribution of variables in a linear regression model show that variations in SARs were explained best by land cover, here expressed as arboreal pollen (AP) percentage (SI Appendix, Fig. S6). Using general additive models on each lake-watershed time series, we found that the probability of positive anomalies in SARs increased as the anomalies in the proportion of AP decreased (P < 0.0001) in 70% of our studied watersheds (Fig. 3F, SI Appendix, Fig. S7, and Table 1). In contrast, SAR time series were less related to the changes in anomalies of precipitation and temperatures (P < 0.01) (Fig. 3 C and D, Table 1, and SI Appendix, Fig. S7) (mean variances explained around 0.6). Sites displaying relatively constant sedimentation rates through time (i.e., benchmark sites, Mann–Kendall (M-K) test ε[−0.2;0.2]) had no clear signs of deforestation in our records (Fig. 2).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 3.

Holocene-scale trends in 12 annually resolved varved sites. (A) Density plot normalized SARs trends for 12 varved lakes. (B) Negative correlation between SARs and AP fraction of the varved sites. (C) Deviance explained in the SAR trends by changes in land cover, precipitation, and temperatures in the GAM models run for each of the 632 ¹⁴C-dated sites. (D) GAMs were run 632 times, and P value results inform the contribution of LCC, precipitation, and air temperature to SARs.

Trend analyses show that despite the heterogeneity in the signal of SARs among sites, some regional patterns were also observed, highlighting different socio-economical histories. For instance, the rise in SARs is later in North America compared to Europe (SI Appendix, Fig. S6) and likely corresponds to the widespread establishment of European agricultural methods shortly after colonization (14). The decrease in sediment fluxes recorded in 23% of the lakes was not explained by land cover and climate variables, and instead may be a result of increased sediment retention due to water uses and river management. Human modification of rivers about 3,000 y ago increased with embankments, dams, watermills, especially in the Chinese and Roman Empires, which could have increased sediment retention (17).

The long-term reconstructions of sediment flux dynamics—i.e., relative changes in sedimentation rates—at global and regional levels presented herein involve some uncertainties due to the nature of ¹⁴C dating and the relatively poor dating resolution at some sites, but nonetheless provide a comprehensive global assessment of the timing of the “low frequency” variations (Fig. 2 A, C, and D). To investigate how short-term variations in land cover could affect SARs, we conducted further analyses on varved records from 12 lakes around the world that had annually dated SARs and pollen records extending over the Holocene (Fig. 3 A and B and SI Appendix, Fig. S8). Consistent with our larger but lower temporal resolution dataset, we found that the SAR trends were anticorrelated with the percentage of AP (Fig. 3B). High rates of change in SAR and pollen records were generally evident in the Early Holocene, followed by an increase of AP percentage associated with decreasing SARs, and then finally during the Late Holocene, we observed increases in SARs and a decrease in AP percentage (SI Appendix, Supplementary Text).

Interestingly, both the varved and lower temporal resolution ¹⁴C-dated cores highlight the importance of climate driven change in SARs for the Early Holocene. This Early Holocene change (i.e., between 12,000 and 8,000 cal. BP) can be explained by the retreat of North Hemisphere ice sheet that progressively gave rise to the creation of most contemporaneous lakes and new lake SAR records. Long records of SARs in lakes found in previously glaciated parts of Europe and North America tended to show high pre-Holocene rates declining to minimum values during the Early to Middle-Holocene (14). For example, AP fraction shows an increase and SARs showed a decrease from 12,000 cal. BP to 10,000 cal. BP in Holzmaar (Germany), or between 10,000 cal. BP and 8,000 cal. BP in the Black Sea as a result of the transient response of vegetation to climate changes (SI Appendix, Fig. S8). During the early to mid-Holocene, the lake-watershed systems seem to progressively reach a stabilization phase associated with the vegetation development, soil maturation, and lake ecosystem stabilization, as evidenced by relatively constant SARs between 8,000 and 4,000 cal. BP. Since the middle of the Bronze Age (3,500 cal. BP), the growing human influence upon erosion is the emergent global trend (9) (Fig. 3 C and D and SI Appendix, Fig. S8).

Collectively, these findings suggest that the abundance of trees in watersheds was the leading factor explaining temporal variations in soil erosion, with anthropogenic deforestation explaining accelerated soil erosion over recent millennia. Our results highlight the importance of large-scale (i.e., in terms of distribution of the lake records globally, but not in terms of the total contributing area) and long-term processes on soil erosion inferred from SARs, and how human activities began to act on these processes much earlier than other signatures of the Anthropocene globally, e.g., human appropriation of the N cycle since 1860 (18). This global-scale analysis of paleolimnological records adds to the growing evidence that humans are simultaneously increasing the river transport of sediment through soil erosion and decreasing this flux to the coastal zone through sediment retention in reservoirs (19). Altogether, our study shows strong evidence that sedimentation in lakes recorded a widespread signal of early increases that are likely the results of land use clearance and increased soil erosion.

• Copyright © 2019 the Author(s). Published by PNAS.