The last 1 million years of the extinct genus Discoaster: Plio–Pleistocene environment and productivity at Site U1476 (Mozambique Channel)

Highlights

• A more intensified water column mixing shown by low values of Florisphaera profunda index occurred at the Mozambique Channel from ~2.4 Ma, resulting in increased abundances of the upper photic zone flora, indicative of nutrient-rich surface water conditions.

• Discoasters declined with global cooling and associated enhancement of surface water productivity in the tropical Indian Ocean across the Plio-Pleistocene.

• Ecological preference of the Plio-Pleistocene Discoaster species resembles that of F. profunda, i.e., warm and oligotrophic surface water conditions.

• The 100-kyr and obliquity signatures suggest a NH driver of the observed variability, whereas variability at the rhythm of precession is interpreted as a tropical Pacific forcing.

Abstract

A detailed paleoenvironment reconstruction from the Mozambique Channel, western Indian Ocean, based on the calcareous nannoplankton assemblages was conducted for the interval between 2.85 and 1.85 Myr. This study covers the period during which the successive extinction of the last five species of discoasters occurred. New productivity data obtained from the abundances of the Discoaster species (Discoaster brouweri, D. triradiatus, D. pentaradiatus, D. surculus, and D. tamalis) and other indicative calcareous nannoplankton taxa showed abundance variations, which were at paced with the 100, 41, and 23 kyr astronomical periodicities. A shift in the productivity and water-column stratification proxies occurred at ~2.4 Ma, after the onset of the Northern Hemisphere glaciation. Here we propose that the variability recorded at International Ocean Discovery Program Site U1476 reflects the interplay between forcing associated with warm tropical Pacific and cold southern ocean influences. The former is shown by consistent occurrence of warm water taxa (Calcidiscus leptoporus, Oolithotus spp., Rhabdosphaera clavigera, Syracosphaera spp., Umbellosphaera spp.), typical of Indonesian Throughflow surface waters. On the other hand, the occurrence of Coccolithus pelagicus indicates the influence of cold, nutrient-rich sub-Antarctic surface waters. A more mixed water column initiated at ~2.4 Ma, and a consequent productivity increase led to the gradual reduction of the Discoaster species, until their extinction at 1.91 Ma. This period was characterized by the low values of the Florisphaera profunda index and high abundances of upper photic zone flora, indicative of nutrient-rich surface water conditions. High productivity at the location during this period could have also been amplified by localized upwelling events driven by the Mozambique Channel eddies.

Introduction

Major climatic variability during the middle part of the Pliocene was first proposed by Shackleton et al. (1984) to have triggered the onset of the Northern Hemisphere (NH) glaciation. Numerous studies have corroborated this suggestion on the basis of paleontological, sedimentological and geochemical records of long climate archives (e.g., Ravelo et al., 2004; Clemens et al., 1996; Christensen et al., 2017). Much of the evidence for this consensus derive from stable carbon and oxygen isotope (δ¹⁸O and δ¹³C) data measured on planktonic and benthic foraminifera (e.g., Raymo et al., 1992; Clemens et al., 1996; Ravelo et al., 2004), suggesting the significance of this phenomenon in the evolution of the Plio-Pleistocene climate. This extreme climatic variability was coupled with global-scale variations in the sea surface temperature (SST) (Clemens et al., 1996) and associated changes in nutrient availability, which could have created a complex oceanographic regime, which in turn controlled plankton distribution in the photic layer. Previous studies suggested that during the late Pliocene (~3 to 2.5 Myr), a shift in marine productivity between the high latitudes and the mid- to low latitudes occurred (e.g., Sarnthein and Fenner, 1988; Bolton et al., 2011). Low values of biogenic silica, CaCO₃, organic carbon, and alkenone accumulation in marine sediments from high latitude regions and high values in the mid- to low latitudes were recorded, suggesting a more productive mid- to low latitude oceans during this time period. These findings have important implications for Plio-Pleistocene climate since marine biological productivity is a key component in the global biogeochemical cycles. Thus studies focusing on the long-term trends in biological responses to climatic variations are essential in understanding the interplay between the local atmospheric processes and ocean circulation over several glacial/interglacial cycles, which is an essential prerequisite in modeling the present and even future climate scenarios.

One of the major contributors to marine primary production, that also plays a key role in both the biological and carbonate pumps are calcareous nannoplankton (nannofossils), a group of single-celled, marine haptophyte algae. These organisms are one of the dominant calcifying plankton groups in the oceans (e.g., Friedinger and Winter, 1987; Westbroek et al., 1993) and within the fossil record, form a major part of its deep-sea sediments (e.g., Flores et al., 1999; Beaufort et al., 2001; Rogalla and Andruleit, 2005). Calcareous nannoplankton lives in the photic layer where light intensity is strong enough to carry out photosynthesis and the nutrient levels are suitable for its growth. The temporal and spatial distributions of calcareous nannoplankton are controlled by latitude (light levels), ocean currents, and the ambient upper ocean nutrient content, salinity, and temperature profiles of the underlying water masses (Winter et al., 1994). They are sensitive to variations in water column characteristics (stratification/mixing), making these organisms potentially ideal recorders of past environmental conditions.

The Plio-Pleistocene is a significant time interval in calcareous nannoplankton evolution history because of the recorded decrease in diversity during this time interval (Bown et al., 2004; Aubry, 2007). High frequency variability in glacial/interglacial temperatures occurred during this interval, with the Pleistocene exhibiting greater variance compared to the late Pliocene (Ravelo et al., 2004; Lisiecki and Raymo, 2005; De Vleeschouwer et al., 2017). The gradual cooling during the transition from the warm Pliocene to the cold Pleistocene (Ravelo et al., 2004) was proposed by Aubry (2007) to have driven the Pliocene nannoplankton turnover and subsequent extinction events. The extinct genus Discoaster is one of the nannoplankton groups that were affected by these extreme climatic fluctuations and transition. Discoaster exhibited a fairly continuous evolutionary development from their first occurrence in the late Paleocene (60 Ma) to the extinction of the last species toward the end of the Gelasian stage (1.93 Ma). Previous studies have suggested that the inception of the NH glaciation during the Pliocene (Shackleton et al., 1984; Raymo et al., 1992; Clemens et al., 1996) led to the successive disappearance of the Discoaster species (e.g., Backman and Pestiaux, 1987; Chapman and Chepstow-Lusty, 1997). The successive extinction of species belonging to this group until its complete demise from the geologic record thus reflects its sensitivity to changes in environmental and oceanographic conditions. While these extinction events are widely documented (e.g., Bukry, 1971; Chepstow-Lusty et al., 1989; Chapman and Chepstow-Lusty, 1997; Raffi et al., 2006; Browning et al., 2017), our knowledge of the ecological preference of the Discoaster species and the environment that they lived in before they disappeared is still limited (e.g, Bukry, 1971; Haq and Lohmann, 1976; Aubry, 1998; Schueth and Bralower, 2015). For instance, the reported diachronous occurrences (Raffi et al., 2006; Schueth and Bralower, 2015) of its member taxa in different ocean basins suggest that the Discoaster extinction cannot be explained by variations in SST alone (e.g., Chepstow-Lusty et al., 1989; Chapman and Chepstow-Lusty, 1997), but is likely a result of a combination of complex environmental parameters (Schueth and Bralower, 2015). There is a general agreement that discoasters have an affinity for warm and oligotrophic water based on assemblage analysis and geochemical evidence (Aubry, 1998; Minoletti et al., 2001; Bralower, 2002; Schueth and Bralower, 2015), although their depth habitat is still poorly understood. In particular, oxygen isotope values of discoasters resembled the planktonic foraminifera (Globorotalia menardii, Dentoglobigerina altispira, Globigerinoides obliquus) record, indicating that they are shallow-dwelling (Minoletti et al., 2001), which is contrary to the findings in other literature that this group prefers the deep photic layer (e.g., Aubry, 1998; Bralower, 2002; Schueth and Bralower, 2015). While this present study cannot completely solve the controversy on their depth habitat, here we provide information on the past variations in environmental and oceanographic conditions in the equatorial Indian Ocean during the Plio-Pleistocene transition that led to the extinction of this long-lived genus.

Here we investigated the temporal distribution of the last five species of this group (D. tamalis, D. surculus, D. pentaradiatus, D. triradiatus, D. brouweri) in the westernmost Indian Ocean using sediments from Site U1476 (Mozambique Channel) collected during the International Ocean Discovery Program (IODP) Expedition 361 – South African Climates (Fig. 1) (Hall et al., 2017b) to reconstruct how the environment and productivity conditions changed toward the end of this lineage. Site U1476 consists of a continuous Plio-Pleistocene sequence of foraminifera-rich or foraminifera-bearing nannofossil ooze (Hall et al., 2017b) and thus offers an exceptional opportunity for high-resolution paleoenvironment and productivity reconstructions. Together with a detailed Plio-Pleistocene calcareous nannofossil biostratigraphy at this site, we present here new records of productivity from the abundances of Discoaster species and compare our results with the downcore abundance record of the extant taxon F. profunda, a widely used productivity proxy, and to other calcareous nannoplankton taxa with established ecological preferences.