Disentangling Natural and Anthropogenic Signals in Lacustrine Records: An Example from the Ilan Plain, NE Taiwan

Abstract

The impact of human activities has been increasing to a degree where humans now outcompete many natural processes. When interpreting environmental and climatic changes recorded in natural archives on historical time scales, it is therefore important to be able to disentangle the relative contribution of natural and anthropogenic processes. Lake Meihua on the Ilan Plain in northeastern Taiwan offers a particularly suitable opportunity to test how human activities known from historical records can be recorded in lacustrine sediment. For this purpose, three cores from Lake Meihua have been studied by a multiproxy approach, providing the first decadal-resolution lacustrine records covering the past 150 years in Taiwan. Profiles of excess 210 Pb, 137 Cs and 239,240 Pu from two short cores (MHL-09-01 and MHL-11-02) allowed a precise chronology to be established. The presence of a yellow, earthy layer with lower levels of organic material coincide with the record of land development associated with the construction of the San-Chin-Gong Temple during AD 1970AD -1982. Furthermore, in the lower part of the cores, the upwards increasing trend of inc/coh, TOC, TOC/TN, and grain size, coupled with the palynological data (increase of Alnus, Mallotus, Trema and herbs) from the nearby core MHL-5A with radiocarbon chronology, suggest that the area surrounding the lake has been significantly affected by agricultural activities since the arrival of Chinese settlers around ∼AD 1874. In sum, this study demonstrates that this suite of lacustrine sediments in northeastern Taiwan has recorded human activities in agreement with historical documents, and that different human activities will leave distinct sedimentological, geochemical, and palynological signatures in the sedimentary archives. Therefore, multiproxy reconstructions are important to capture the complex nature of human-environmental interactions. A better understanding of the weathering and erosion response to human activities can also provide useful information for sustainable land-use management.

Radionuclide 239+240Pu for Dating and Sedimentation Rate in Chinese Lakes

“…To the best of our knowledge, this study has summarized all the available published references in Chinese lakes (Table , Figure , and Table S1) and found 24 sediment cores to study 239+240 Pu inventory. ,,− The 239+240 Pu inventory in the lake sediment core is calculated following eq below. I = prefix∑ i = 1 n ( B i × C i × D i ) where I is the inventory (Bq m –2 ) of 239+240 Pu in the lake sediment core as shown in eq , n is the number of subsampled layers, i is the number of subsampling layers of the lake sediment core, B i is the dry density of the i -th layer sediment core (g cm –3 ), C i is the 239+240 Pu activity in the i -th layer of the sediment core (mBq g –1 ), and D i is the thickness of the i -th layer (cm).…”

“…By summarizing available literature, we have collected a total of 54 sediment cores of 239+240 Pu in the global lakes, with a particular focus on China. As shown in Figure , there are 20 lakes (black dots) and 28 239+240 Pu sediment cores in China, with all the data derived from published papers ,,− (see more information in Tables S1 and S2). Lake 239+240 Pu samples outside China are randomly distributed on different countries, namely, Brazil, Canada, Finland, Japan, Lithuania, Mexico, New Zealand, Norway, Russian, and Switzerland − (See more information in Tables S1 and S3).…”

“…However, there are only 20 lakes and 28 sediment cores to use 239+240 Pu for dating and source recognition in China. 1,4,12−27 Many earlier studies have investigated the vertical distribution of the 239+240 Pu activity and 240 Pu/ 239 Pu atom ratio in lake sediment cores, but they examined only one sediment core [12][13][14][15][20][21][22]26 or a limited number of sediment cores in a specific lake and reservoir. 16,18,19,25 In particular, the relationship between the position of the 239+240 Pu peak and the sedimentation rate in different lake sediment cores still lacks comprehensive research, and the source of 239+240 Pu in Chinese lake sediment cores needs to be summarized.…”

“…China is a country with more than 24,800 lakes, which is an ideal regional research 239+240 Pu distribution and deposition. However, there are only 20 lakes and 28 sediment cores to use 239+240 Pu for dating and source recognition in China. ,,− Many earlier studies have investigated the vertical distribution of the 239+240 Pu activity and 240 Pu/ 239 Pu atom ratio in lake sediment cores, but they examined only one sediment core − ,− , or a limited number of sediment cores in a specific lake and reservoir. ,,, In particular, the relationship between the position of the 239+240 Pu peak and the sedimentation rate in different lake sediment cores still lacks comprehensive research, and the source of 239+240 Pu in Chinese lake sediment cores needs to be summarized. Furthermore, the feasibility of using the convection dispersion equation (hereafter, CDE) model to study the migration rate or apparent convection velocity of 239+240 Pu in a lake sediment core needs to be further discussed. , …”

High-resolution records of anthropogenic activity and geohazards from the reservoir of Sun Moon Lake, Central Taiwan

“…It is clearly shown that there is a distinctive boundary separating the whole record into two parts: The upper one is characterized with higher MS, higher contents of typical elements (e.g., Ti, K, Fe) associated with terrestrial silicates and lower Mo inc/coh, as well as low values in b*, signifying less yellowish or brownish in color. The Mo inc/coh (incoherent/coherent ratio) indicates organic content (Chawchai et al, 2015;Huang et al 2016a). The boundary marks a major change in deposition, from the previous wetland, peat-rich sediments to the lake facies when the dam was completed in 1934 CE.…”