The Holocene paleo-environmental history of the Gangkou River estuary, Hengchun Peninsula, Taiwan

Abstract

The Taiwan coastal areas are facing predicted sea-level rise effects in the global climate change context. Better understanding of the mechanisms and forcing factors driving the geomorphological evolution of Taiwan's coastline is essential. We highlight the potential of small river estuaries as archives of coastal evolution and provide a paleo-environmental reconstruction of the Gangkou River estuary based on results from a combined methodological approach using geomorphological, sedimentological, and geochronological analyses. A consistent chronology was established by combining Optically Stimulated Luminescence (OSL) dating, as well as radiocarbon dating techniques. A distinct beach-rock layer allowed firm correlation of six investigated coastal profiles. Eustatic sea level change was identified as the primary coastal and estuarine landscape development forcing factor in the study area. Tectonic forcing could also be detected from the investigated sites, but has to be regarded as a subordinate secondary forcing factor. The Gangkou River estuary developed in the early Holocene at about 8 ka and was fully developed by the time the Holocene sea-level maximum was reached. With the sea-level dropping until modern times, estuarine zone relocation and a transition to terrestrial processes was initiated, leading to development of the current coastal dune system since about 2.5 ka. Major dune building phases correlate with the climate change associated with the Little Ice Age (LIA) event and stabilised towards recent times, which is in accordance with regional and over-regional coastal Aeolian records in Taiwan.

Holocene fluvial landscape evolution driven by sea level and tectonic controls in the Gangkou River, Hengchun Peninsula

“…7. From our field investigations, we found that the sediment underlying the coastal sand dunes is a mud layer dated to ~8 ka (Lüthgens et al 2018). The high river terrace on the right bank of the Gangkou River in the GY area was chosen for geological drilling.…”

“…The river mouth is usually blocked by barriers, which typically produce a swath of relatively still water extending landward for several hundred meters to 1 km. It is worth mentioning that Lüthgens et al (2018) investigated the coastal landscape south of the Gangkou River mouth. They reported an ~8 ka estuarine muddy bed, showing that the river flowed ~600 m south of its present position or, more likely, formed an estuary at least 700 m wide (including the present river mouth), apparently caused by the Holocene sea level rise.…”

“…The fact that this date is derived from an elevation 5 m higher than the ~4.5 ka beachrock reveals northward tectonic tilting. Moreover, by using optically stimulated luminescence (OSL) dating methods, Lüthgens et al (2018) showed an episode of coastal and fluvial aggradation starting at approximately 2.5 ka, in conjunction with the building of the beach/sand dune on the ~4.5 ka beachrock and the genesis of low-level (< 5 m high) terraces along the river. Therefore, the lack of buffering landscape features, such as alluvial plains, alluvial fans, fluvial landforms, and river terraces, along the Gangkou River was likely directly influenced by changes in relative sea level.…”

“…The results were then calibrated by using CALIB 7.04 to correct the ages (Reimer et al 2013). Several previously published radiocarbon ages from Lüthgens et al (2018) were recalibrated with the mentioned method and used here.…”

“…During recent millennia, the uplift rates of the HP have varied widely both in space and time. Chen and Liu (1993) and Lüthgens et al (2018) show that the uplift rates of marine terraces on the eastern HP are 2.2 -2.1 and 0.9 -1.4 mm yr -1 , respectively, while Chen and Liu (1993), Chen et al (2005b), and Chen et al (2014) concluded that the western part of the HP has uplift rates greater than 3 mm yr -1 . Under these circumstances, it is most likely that the terraces developed in the Gangkou River were probably influenced by the relative sea level change directly without delay or interruption from any buffer zone in between.…”