Abstract
The sensitivity of magnetic properties, which characterize the mineralogy, concentration, and grain size distribution of magnetic minerals, to environmental processes may provide useful information on paleoenvironmental changes in estuarine environments. Magnetic property studies of estuaries are less common than other environments and, due to the west coast of South Korea having an abundance of estuaries, it provides a good place to study these processes. In this study, we analyzed a variety of magnetic properties based on magnetic susceptibility, hysteresis parameters, progressive acquisition of isothermal remanent magnetization and first-order reversal curve data from a Holocene muddy sediment core recovered from the Yeongsan Estuary on the west coast of South Korea. We examined diagenetic effects on magnetic properties and tested their availability as proxies of paleoenvironmental change. The presence of generally low magnetic susceptibility, ubiquitous greigite-like authigenic magnetic component, and very fine magnetic particle occurrence suggested that the analyzed sediments had undergone considerable early diagenetic alteration. Electron microscopic observations of magnetic minerals support this suggestion. Our results confirm that the use of initial bulk susceptibility as a stand-alone environmental change proxy is not recommended unless it is supported by additional magnetic analyses. We recognized the existence of ferromagnetic-based variabilities related to something besides the adverse diagenetic effects, and have examined possible relationships with sea-level and major climate changes during the Holocene. The most remarkable finding of this study is the two distinct intervals with high values in magnetic coercivity (Bc), coercivity of remanence (Bcr), and ratio of remanent saturation moment to saturation moment (Mrs/Ms) that were well coincident with the respective abrupt decelerations in the rate of sea-level rise occurred at around 8.2 and 7 thousand years ago. It is then inferred that such condition with abrupt drop in sea-level rise rate would be favorable for the abrupt modification of grain size distribution toward more single-domain-like content. We modestly propose consideration of the Bc, Bcr, and Mrs/Ms variability as a potential indicator for the initiation/occurrence of sea-level stillstand/slowstand or highstand during the Holocence, at least at estuarine environments in and around the studied area.
Highlights
Magnetic minerals are common constituents of a wide range of sediments and are sensitive to the physicochemical conditions of their surrounding environment (Thompson and Oldfield, 1986; Verosub and Roberts, 1995; Dekkers, 1997; Maher and Thompson, 1999; Evans and Heller, 2003; Torii, 2005; Liu et al, 2012)
There was exactly no horizon where bulk and extract subsamples both were measured for these magnetic property values in this study, it may be supported in part by the fact that, in the interval of 7.90–8.00 m depth in core, their values on bulk and extract subsamples were apparently similar
We analyzed 3.2–to–19.8-m depth interval of a 20m-long Holocene muddy sediment core recovered from the Yeongsan Estuary, South Korea, using mineral magnetic measurements with sediment particle size and total organic carbon (TOC) content to characterize downcore variations in a variety of magnetic properties
Summary
Magnetic minerals are common constituents of a wide range of sediments and are sensitive to the physicochemical conditions of their surrounding environment (Thompson and Oldfield, 1986; Verosub and Roberts, 1995; Dekkers, 1997; Maher and Thompson, 1999; Evans and Heller, 2003; Torii, 2005; Liu et al, 2012). It is generally thought that sedimentation during Holocene at estuaries and coastal lines are sensitive to their catchment (terrestrial) environmental changes and extreme hydrologic events such as heavy rainfalls and storms, which are closely associated with regional and global climate changes, and regional sea-level change (e.g., Lim et al, 2017, Lim et al, 2019). Such estuarine‒coastal sediments are of potential use for reconstructing records of paleoenvironmental changes during Holocene. During the Holocene, these western coastal areas would have experienced dramatic environmental changes due to e.g., sea-level changes such as the Holocene transgression (Stanley and Warne, 1994; Kim and Kennett, 1998), the El Niño Southern Oscillation (ENSO) activity (e.g., Lim et al, 2017; Lu et al, 2018; Lim et al, 2019), the eastern Asian Monsoon (EAM; e.g., Chang, 2004; Dykoski et al, 2005; Selvaraj et al, 2007), and the Holocene climate optimum (HCO; e.g., An et al, 2000; Zhou et al, 2016; Park et al, 2019)
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