High‐Resolution Coccolithophore Morphological Changes in Response to Orbital Forcings During the Early Oligocene. (31st March 2023)

• Record Type: Journal Article
• Authors: Ma, Ruigang; Jin, Xiaobo; Liu, Chuanlian
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 24:Number 4(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-03-31
• Subjects: coccolithophore -- morphology -- high‐resolution -- nutrient -- Oligocene; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2022GC010746
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: The global climate of the early Oligocene was characterized by initiated Antarctic glaciation and meridional overturning circulation, which then led to profound eutrophication in the upper ocean. Generating a high‐resolution coccolith record helps to understand the responses of marine phytoplankton to the newly established environment. Using highly resolved (∼6 kyr time‐resolution) marine sediment samples from Deep Sea Drilling Project Site 522 in the South Atlantic Ocean, we conducted a comprehensive morphological study on coccoliths from the genera Reticulofenestra, Dictyococcites, and Coccolithus, which dominated the study interval between ∼33.1 and 32.8 Ma. Our results showed that the size variations of the three measured genera were significantly correlated ( p < 0.01) with each other, indicating homogeneous responses to the environmental changes. Moreover, spectrum analysis on integrated morphologic data of all measured coccoliths showed distinct obliquity (∼40‐kyr) and precession (∼23‐kyr and ∼18‐kyr) cycles. We suggest that these variations were mainly driven by temperate, short‐term ecological fluctuations, which periodically altered the nutrient conditions in the common living habitats of the studied coccolithophores. We proposed two tentative explanations focusing on the obliquity signal. First, the cyclic variation could result from obliquity‐modulated changes in ice volume and variations in ocean circulation intensity, which influenced nutrient export from deep waters to the upper ocean. Alternatively, the changes in coccolith size may indicate the strength of seasonality that influenced upper ocean mixing on the west coast of South Africa. Plain Language Summary: Phytoplankton sizes yield important information to understand changes in marine ecosystems. Coccoliths, the fossil remains of unicellular algae coccolithophores, are preserved in marine sediments and thus are ideal for recording phytoplankton size changes in the past. We present a high‐resolution coccolith record spanning a period of ∼300, 000 years in the early Oligocene (∼33 million years ago). Our data showed changes in coccolithophore sizes, corresponding to Earth's orbital cycles. We propose that the increased size of coccolithophore corresponded to nutrient‐rich conditions. Strengthened ocean circulation or enhanced wind‐driven mixing that is related to Earth's orbital cycles are two possible driving mechanisms for the nutrient increase in the upper ocean. Our study provides direct evidence of Earth's orbital forcings on the marine ecosystem in the early Oligocene when Antarctica started to be covered by a continental ice sheet. Key Points: This study presents a high‐resolution morphological record of early Oligocene coccolithophores in the South Atlantic Three measured coccolithophore genera showed homogenous responses to short‐term ecological fluctuations Ocean circulation and seasonality modulated by orbital forcing might have controlled the nutrient availability in the upper ocean