Holocene evolution in weathering and erosion patterns in the Pearl River delta

Abstract

Sediments in the Pearl River delta have the potential to record the weathering response of this river basin to climate change since 9.5 ka, most notably weakening of the Asian monsoon since the Early Holocene (∼8 ka). Cores from the Pearl River delta show a clear temporal evolution of weathering intensity, as measured by K/Al, K/Rb, and clay mineralogy, that shows deposition of less weathered sediment at a time of weakening monsoon rainfall in the Early‐Mid Holocene (6.0–2.5 ka). This may reflect an immediate response to a less humid climate, or more likely reduced reworking of older deposits from river terraces as the monsoon weakened. Human settlement of the Pearl River basin may have had a major impact on landscape and erosion as a result of the establishment of widespread agriculture. After around 2.5 ka weathering intensity sharply increased, despite limited change in the monsoon, but at a time when anthropogenic pollutants (e.g., Cu, Zn, and Pb) increased and when the flora of the basin changed. 87Sr/86Sr covaries with these other proxies but is also partly influenced by the presence of carbonate. The sediments in the modern Pearl River are even more weathered than the youngest material from the delta cores. We infer that the spread of farming into the Pearl River basin around 2.7 ka was followed by a widespread reworking of old, weathered soils after 2.5 ka, and large‐scale disruption of the river system that was advanced by 2.0 ka.

A dissolved cobalt plume in the oxygen minimum zone of the eastern tropical South Pacific

“…In contrast to depleted Co along the South American shelf, the Co / Al ratio in shelf sediments from the western margin of the Pacific appears crustal (Table 2). Holocene records from the Pearl River delta and shelf slope in the South China Sea ∼ 20 • N (Hu et al, 2012(Hu et al, , 2013 show mean Co / Al ratios of 2.2 and 2.1 × 10 −4 g g −1 , respectively, similar to sediments from the Gulf of Papua at 9 • S (2.3 × 10 −4 g g −1 , Alongi et al, 1996). Crustal Co / Al in these sedimentary systems implies that most of the Co provided from fluvial sediment delivery either does not dissolve or is quickly reburied by water column Mn oxidation, rates of which can be very high in estuaries and coastal seas (Moffett and Ho, 1996;Moffett, 1994;Sunda and Huntsman, 1987;Sunda and Huntsman, 1990).…”

The cobalt cycle in the Tropical Pacific Ocean

“…Holocene records from the Pearl River delta and shelf slope in the South China Sea ~20° N (Hu et al, , 2013 show mean Co/Al ratios of 2.2 and 2.1 x 10 -4 g g -1 , respectively, similar to sediments from the Gulf of Papua at 9° S (2.3 x 10 -4 g g -1 , . Crustal Co/Al in these sedimentary systems implies that most of the Co provided from fluvial sediment delivery either does not dissolve or is quickly reburied by water column Mn oxidation, rates of which can be very high in estuaries and coastal seas Moffett, 1994;.…”

Chemical Weathering Intensity and Terrigenous Flux in South China during the Last 90,000 Years—Evidence from Magnetic Signals in Marine Sediments

“…Marine terrigenous detrital sediments record information from chemical weathering of the continental crust, which is directly related to climate (Clift, 2006;Hu et al, 2012Hu et al, , 2013Zhao and Zheng, 2015). The intensity of weathering can be represented by some chemical element ratios in the marine sediments, such as Al/Ti, Al/K (Kronberg et al, 1986;Wei et al, 2004;Zhao and Zheng, 2015).…”

“…However, some researchers argue that strong physical erosion may be related to a cold climate owing to faster run-off in the absence of drainage capture . Major and trace elements taken from sediments of the South China Sea have been used to study links between weathering and monsoon paleo-intensity (Wei et al, 2004(Wei et al, , 2006Hu et al, 2012Hu et al, , 2013. However, these studies are inadequate to clarify the relationships between chemical weathering, physical erosion, and monsoon variations.…”