Elsevier

Science Bulletin

Volume 61, Issue 11, June 2016, Pages 897-910
Science Bulletin

Article
Earth Sciences
Lake geochemistry reveals marked environmental change in Southwest China during the Mid Miocene Climatic Optimum

https://doi.org/10.1007/s11434-016-1095-xGet rights and content

Abstract

The Mid-Miocene Climatic Optimum (MMCO; ~ 15–17 Ma) was one of the short-term climatic warm events that punctuated the Cenozoic long-term cooling trend. Because there are very few terrestrial records of this event, most of our understanding comes from marine cores. In this report, we first present new palaeomagnetic data that revises the dating of our 400 m-thick lacustrine section in Wenshan (Yunnan), previously thought to be Late Miocene. These new data suggest an older age, ca. 15.2–16.5 Ma, coinciding with the MMCO. We measured δ13C on bulk organic matter (δ13Corg), total organic carbon (TOC), total nitrogen (TN) and C/N ratios at a high sample resolution to: (1) reconstruct the palaeoenvironmental changes in the lake catchment area, and (2) infer mechanisms responsible for these changes. Our results show that all four geochemical parameters demonstrate that a strong environmental change occurred around the middle of the section, shortly after the C5Cn/C5Br geomagnetic reversal and the Early/Middle Miocene boundary at 15.97 Ma. We propose that the environmental shift may be due to a combination of a change in climate, which became cooler, together with a change in organic matter cycling within the lake. This study provides a new insight into the MMCO and demonstrates that although the MMCO was generally a warm event, it was also a time of climatic instability and abrupt environmental changes.

Introduction

Marine isotopic data have revealed that the long-term cooling trend of the Cenozoic was punctuated by short-term climatic events [1]. The Miocene climate exhibited marked changes including the Mid-Miocene Climatic Optimum (MMCO, ~ 15–17.0 Ma [2]) during which temperatures were inferred to be ~ 3–8 °C higher than pre-industrial level 3., 4.. The mechanisms leading to the MMCO are still debated [2], but evidence points to an increase in the concentration of atmospheric CO2 [4].

Until now, the MMCO has been mainly investigated using marine isotopic data 1., 2., 5., 6., since terrestrial outcrops are relatively scarce. In Antarctica, a palynological study and leaf wax geochemistry revealed a warmer and wetter climate than today, and demonstrated that Antarctica could support a tundra vegetation 7., 8.. In China, a lithological study conducted in the northeast Tibetan Plateau and a palynological investigation in Tian Shan (northwest China) show evidence of a warm and wet period during the MMCO 9., 10., demonstrating that the lower latitude continental areas were also affected by the MMCO. However, it is still unclear how subtropical environments responded to the MMCO due to the scarcity of suitably dated outcrops.

In Yunnan (southwest China), the evolution of the Miocene climate has been extensively investigated using different floral proxies, such as pollen 11., 12. and plant mega-fossils 13., 14., 15., 16., 17., indicating that Yunnan was already under a monsoonal climate in the Miocene 11., 14.. We have previously reported palynological results from a 400 m-thick sedimentary succession in Wenshan, southeast Yunnan, to reveal how the vegetation dynamics responded to a monsoonal climate [11]. Lakes are environmentally sensitive and so are ideal to study past climatic fluctuations [18]. For example, lake sediment geochemistry has been widely used to explore environmental changes on the Tibetan Plateau during the Miocene 19., 20. and in Yunnan during the Quaternary 21., 22., 23.. However, there is very limited research using terrestrial geochemistry to investigate environmental changes in Yunnan during the Miocene, and the MMCO in particular.

In the present study, we present new magnetostratigraphic data to date the sedimentary succession. Then, we employ lake sediment organic geochemistry with high sampling density to: (1) further investigate palaeoenvironmental changes in the lake catchment area in Wenshan using geochemical proxies (δ13 C on bulk organic matter, total organic carbon, total nitrogen, and C/N ratio); and (2) discuss the factors responsible for these palaeoenvironmental changes.

Section snippets

Geological setting

Our study section of the Wenshan palaeo-lake is located on the South China Block, close to the Ailao Shan-Red River Fault, which underwent a mid-Tertiary (~35–17 Ma ago) sinistral shear displacement before being reactivated as a right lateral system during the Pliocene [24]. The time when this fault was active is still debated, but evidence shows that there was movement during the Miocene and this may have influenced the topography of Wenshan at the time of sediment accumulation 25., 26.. A

Chronology construction

The AF demagnetization method used in this study was capable of isolating the characteristic remanent magnetization (ChRM) after removal of soft secondary components of magnetization. Representative demagnetization diagrams are shown in Fig. 3. For most samples, the high-stability ChRM component was separated between 30 and 70 mT. The behaviours indicate that magnetite dominates the ChRM carriers in the Wenshan lacustrine sediments. After stepwise AF demagnetization, a total of 217 specimens

Environmental significance of geochemical proxies

During sediment transport, a significant proportion of organic matter is usually lost through cycling 47., 48., but the original signatures of C/N and δ13Corg of bulk organic matter are not markedly affected in aqueous environments 47., 48., 49.. Small variations are observed in the isotopic composition of soil organic matter during litter decay [48], but this is likely to have little impact in Wenshan compared to the large range in δ13Corg values. After deposition at the bottom of the lake,

Conclusions

We studied a 400 m-thick lacustrine section in Wenshan, southeast Yunnan, to explore environmental change in the lake catchment area. First, we revised the geological age of the section, previously thought to be the Late Miocene. Magnetostratigraphic dating suggests an older age of ca. 15.2–16.5 Ma, and that deposition was during the MMCO. Then, we measured four geochemical indices (δ13Corg, TOC, C/N and TN) in a sedimentary section of the palaeo-lake to better understand environmental changes

Conflict of interest

The authors declare that they have no conflict of interest.

Acknowledgments

The authors are grateful to two anonymous reviewers for their constructive comments, which significantly improved the manuscript. The authors thank fellow members of staff of the Palaeoecology group in Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences for assistance during sample collection and productive critical discussions; Professor Yun Fu from the Central Laboratory of Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences for assistance during the TN

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