Mechanism and implications of upwelling from the Late Ordovician to early Silurian in the Yangtze region, South China

Oceanic upwelling is an important process in ocean cycling and has implications for climate change and ocean productivity. During the Late Ordovician to early Silurian (O/S) transition, many dramatic geologic events occurred, including the Hirnantian Glaciation, enhanced organic matter accumulation, and upwelling events. Upwelling is thought to be closely linked to contemporary geologic events; however, the internal relationships and mechanisms involved remain poorly understood due to a reliance on qualitative evidence and low-resolution data. To better determine the presence and development of upwelling, as well as its underlying mechanisms and potential implications for environmental changes during the O/S transition, we report new evidence and insights based on data for South China. Detailed petrographic and geochemical analyses of a well-sequenced drill core from the Lower Yangtze region are presented and compared with other extensively examined sections from the Upper Yangtze region. We show that the excessive biogenic silica content associated with certain geochemical fingerprints (i.e., Cd/Mo, Co x Mn, and Co-EF x Mn-EF) offer compelling evidence for widespread upwelling in the Yangtze region during the O/S transition. This upwelling was strong during the late Katian to Hirnantian, significantly weaker during the early Rhuddanian, and was generally more intense in the relatively deep waters of the Yangtze region. The glacially-influenced temperature difference between the poles and equator, as well as longshore trade winds, may have been the drivers of this widespread upwelling. The upwelling event may have increased surface productivity, driving the accumulation of organic matter in the Wufeng Formation in South China. Moreover, the upwelling event can be regarded as a broad indicator of Late Ordovician glaciation and may be the geochemical basis for the Boda warm Event that occurred prior to Hirnantian cooling.

Automated summary

Studies have found widespread oceanic upwelling in the Yangtze region during the Late Ordovician to early Silurian transition, possibly driven by glacially-influenced temperature differences between the poles and equator and longshore trade winds. This upwelling event may have led to organic matter accumulation in South China and could be considered a geological indicator of Late Ordovician glaciation and the Boda warm Event prior to the Hirnantian cooling.