# Last Abundant Appearance Datum of Hemidiscus karstenii driven by climate change

Xavier Crosta, Sunil Kumar Shukla, Olivier Ther, Minoru Ikehara, Masako Yamane, Yusuke Yokoyama

► To cite this version:

Xavier Crosta, Sunil Kumar Shukla, Olivier Ther, Minoru Ikehara, Masako Yamane, et al. Last Abundant Appearance Datum of Hemidiscus karstenii driven by climate change. Marine Micropaleontology, 2020, 157, pp.101861. 10.1016/j.marmicro.2020.101861. hal-02525916

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# Last Abundant Appearance Datum of *Hemidiscus karstenii* driven by climate change

Xavier Crosta<sup>a,*</sup>, Sunil Kumar Shukla<sup>b</sup>, Olivier Ther<sup>a</sup>, Minoru Ikehara<sup>c</sup>, Masako Yamane<sup>d,e</sup>, Yusuke Yokoyama<sup>d,f</sup>

<sup>a</sup> UMR 2805 EPOC, Université de Bordeaux, 33615 Pessac cedex, France (xavier.crosta@ubordeaux.fr)

<sup>b</sup> Birbal Sahni Institute of Paleosciences, 53 University Road, Lucknow 226 007, India

<sup>c</sup> Center for Advanced Marine Core Research, Kochi University, Japan

<sup>d</sup> Atmosphere and Ocean Research Institute, University of Tokyo, Japan

<sup>e</sup> Institute for Space-Earth Environmental Research, Nagoya University, Japan

<sup>f</sup> Biogeochemistry Program, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Japan

## Abstract

The Last Abundant Appearance Datum of the *endemic* Southern Ocean diatom *Hemidiscus karstenii* Jousé is dated at around 190,000 years Before Present, which provides a robust and convenient biostratigraphic marker to Late Pleistocene paleoceanographic studies. However, no *prior* studies *have* sought to understand what processes *may have driven H. karstenii* to extinction. We here present the first morphometric investigation of *H. karstenii* over Marine Isotope Stages (MIS) 10 to 6 (355,000 years to 150,000 years). The combination of these data to records of *H. karstenii* absolute abundances, *Fragilariopsis kerguelensis* biometry and absolute abundances, and southern high-latitude air and ocean temperatures over the past 355,000 years suggests that *H. karstenii* populations reduced *their* mean size and productivity in response to the rapid drop in temperatures across the MIS 7 to MIS 6 glacial inception. *This prolonged cooling event* may have pushed *H. karstenii* to its adaptive limit until it could not sustain physiological viability. The repetitive occurrence of very cold *events* during the early MIS 6 may have additionally prevented *surviving populations of H. karstenii* from re-establishing abundant communities leading to extinction late in MIS 6.

## 1. Introduction

The diatom *Hemidiscus karstenii* Jousé is an endemic species to the Southern Ocean that appeared ~8,9 millions of years (Myrs) ago and seemingly *went* extinct ~190,000 years (190 kyrs) ago (Barron, 2003) during the transition between Marine Isotopic Stage (MIS) 7 to MIS 6. Studies reporting *H. karstenii* presence in Southern Ocean sediments suggested a Subantarctic habitat of this species (Fig. 1) whose abundances reached *their highest levels* during interglacial periods and lowest during glacial periods. The final drop occurred in a few thousands of years at ~190 kyrs, with abundances of 20-30% during the late MIS 7 to less than 3% during the early MIS 6 (Burckle et al., 1978; Burckle and Burak, 1988; Fenner et al., 1991; Gersonde and Bárcena, 1998; Crosta et al., 2004). This Last Abundant Appearance Datum (LAAD), or Last Appearance Datum (LAD), has been widely used as a robust biostratigraphic marker (Graves, 1982; Baldauf and Barron, 1991; Nees et al., 1999; Pudsey et al., 2000; Zielinski and Gersonde, 2002; Giulani et al., 2011; Presti et al., 2011; Lee et al., 2012; Tolotti et al., 2018).

The LAAD of *H. karstenii* occurred during a period of high diatom species diversity (Lazarus et al., 2014; Benoiston et al., 2017), and not in a period of enhanced extinction. Notably higher Southern

<footnote>* Corresponding author</footnote>
Ocean endemic diatom taxonomic turnover rates have been documented between 5 Myrs and 1.6 Myrs (Barron, 2003). In this context, the reasons underlying the disappearance of *H. karstenii* are not understood and non-exhaustively may involve (1) evolution into a different lineage, (2) competition with other organisms, (3) predation, (4) taphonomic processes and (5) lack of plasticity to acclimate to environmental stresses. Issues of mis-identification might not be applicable to this easily identifiable species. Evolution in a different lineage would imply the coincident appearance of one or several related morphotypes, as is observed in the diatom genus *Rhizosolenia* (Sörhannus et al., 1978). However, no corresponding species, nor any other diatom species, arose at around ~190 kyrs. In the Southern Ocean, the closest morphological species *Hemidiscus cuneiformis* appeared 11,5 Myrs ago and is still extant, while species from the neighbor genus *Actinocyclus* have totally different temporal ranges (Barron, 2003). Changes in predation and/or in competition with other phytoplankton organisms cannot be ruled out but might have resulted in the coincident disappearance of other diatom species having similar ecological and environmental requirements to *H. karstenii*. Similarly, any taphonomic influence on this morphologically robust diatom species would have also impacted other diatom species. However, *H. karstenii* is the only diatom species that disappeared at ~190 kyrs, bracketed by the extinction of *Rouxia constricta* at ~290 kyrs and *R. leventerae* at 140 kyrs (Zielinski et al., 2002).

In the Southern Ocean, most diatom species have relatively narrow ecological preferences. However, some of them are found from the seasonal sea-ice zone to the Subantarctic Zone (Armand et al., 2005; Crosta et al., 2005a; Romero et al., 2005) suggesting a strong intra-specific plasticity that allows them to acclimate to different and highly variable environmental conditions (Kitchell et al., 1986; Racki, 1999; Godhe et al., 2017). Nonetheless, specimens living in high-stress environments are generally smaller than their counterparts thriving at their optimal ecological range. Such dwarfism, called the Lilliput Effect, has been observed in many terrestrial and marine organisms from mammals to foraminifera (Keller and Abramovich, 2009; Wade et al., 2009) and diatoms (Cortese and Gersonde, 2007; Shukla et al., 2016). More specifically, *Fragilariopsis kerguelensis* and *Thalassiosira lentiginosa*, two diatom species endemic to the Southern Ocean, were shown to grow smaller valves when temperatures were too low, sea ice was too consolidated and macro- and micro-nutrient stocks were too depleted (Crosta et al., 2009; Cortese et al., 2012; Shukla et al., 2013, 2016; Nair et al., 2019). We here build on the relationship between high-stress environments and diatom size reduction to document whether the glacial conditions prevailing during MIS 6 could have contributed to *H. karstenii* disappearance. To this point, we performed morphometric measurements on *H. karstenii* in core DCR-1PC, retrieved below the modern Subantarctic Front in the southwestern Indian Ocean (Fig. 1), over the 355-150 kyrs period, thus encompassing two glacial-interglacial cycles. We compare this first record to a new *F. kerguelensis* size record and a new sea-surface temperature record from the same core, both covering the last 355 kyrs. These latter records allow documentation of changes in environmental conditions in the southwestern Indian sector and the response of a well-studied diatom species as a basis to inferences made on *H. karstenii* plasticity.

## 2. Material and methods

### 2.1. Core description and age model

Sediment core DCR-1PC was obtained from the southern slope of Del Caño Rise (46°01.34'S – 44°15.24'E – 2632 m of water depth) in the Indian sector of the Southern Ocean during the oceanographic expedition Hakuho-maru KH-10-7 during the austral summer 2010-2011. The site of DCR-1PC is located below the modern Subantarctic Front (SAF) where modern mean summer sea-surface temperatures (SST) are ~7.50°C (Fig. 1). Piston core DCR-1PC measures 10.20 meters and is
composed of homogenous nannofossil oozes deposited during warm periods and homogenous diatom oozes buried during cold periods.

The age model of core DCR-1PC is based on seven radiocarbon dates and tuning of the diatom-based SST record to the deuterium record of the well-dated EPICA Dome C ice core (Bazin et al., 2013). The Accelerated Mass Spectrometry (AMS) radiocarbon dates ($^{14}$C) were performed on monospecific samples of the planktic foraminifera *Globigerina bulloides* and *Neogloboquadrina pachyderma* sinistral. Radiocarbon samples were treated according to the protocol described in Yokoyama et al., (2007, 2010) with target graphite measured at the AMS facilities Micro Analysis Laboratory, Tandem accelerator (MALT), and Atmosphere and Ocean Research Institute (YAUT), at the University of Tokyo. The $^{14}$C dates were calibrated using CALIB7.02 software using the Marine 13 calibration curve (Reimer et al., 2013) after a marine reservoir age correction of 890±100 years as advised for this region (Butzin et al., 2005) (Fig. 2). Beyond the radiocarbon range, the comparison of the summer SST record to EDC deuterium record provided twenty additional tie-points (Fig. 2). The depth-age conversion was done by linear interpolation between successive control points. With the chronology developed here the LAAD of *H. karstenii* is independently dated at ~190 kyrs BP in agreement with published LAAD ages.

## 2.2. Diatom analyses

Sediment processing and slide preparation was improved from the method described in Rathburn et al. (1997). A known weight of dry sediment was deposited in a beaker. The organic matter was removed by adding 20 ml of oxidative solution (870 ml of H₂O₂ at 35% + 130 ml of distilled water + 42 g of Na₄O₇P₂-10H₂O for 1 l solution) to the beaker held in a ~65°C water bath until bubbling ceased. The solution was then rinsed three times by adding distilled water and centrifugation during 7 min at 1200 rpm. The aliquot was then re-deposited in the beaker and carbonates were removed by adding 20 ml of HCl at 30%. The sample was again rinsed three times and poured back into the beaker. The sample was diluted to ca. 50 ml (or any other volume depending the diatom concentration expected). Two drops (or any number of drops depending on the diatom concentration expected) were pipetted from the solution, which had been previously gently stirred, and deposited into a petri dish. The petri dish contained a cover slip, glued to the bottom of the dish by a drop of water, and was half filled with distilled water. Immediately after the drops had been deposited in the petri dish, this one was completely filled with distilled water to ensure a good dispersal of the material. After 30 min, a wet wool thread was added to pump the water out of the petri dish overnight to drain and dry. The dry coverslip was then glued to a slide using NOA61 adhesive that hardens under UV light.

Taxonomic criteria and identification of *H. karstenii* followed Ciesielski (1983), Fenner et al. (1991) and Zielinski and Gersonde (2002).

Diatom counting followed the rules described in Crosta and Koç (2007). Around 350 diatom valves were counted in each sample at 1000x magnification on a Nikon Eclipse 80i phase contrast microscope. Diatoms were identified to species or species group level. Absolute abundances of diatoms were calculated following the equation detailed in Crosta et al. (2008). The relative abundance of each species was determined as the fraction of diatom species against total diatom abundance in the sample. Diatom counting was performed every ~1 cm over the upper 375 cm (~140 kyrs) and across terminations III and IV, providing a mean temporal resolution of ~450 years. Resolution was decreased to ~4 cm in glacial stages, providing a mean temporal resolution of ~2000 years.

Diatom biometry was conducted on the slides used for diatom census counts. One hundred specimens of *H. karstenii* and of *F. kerguelensis* were measured manually using NIS Element D imaging software on a computer connected to the microscope through a live camera. For both species, the apical length and transapical width were measured (Fig. 3). Reproducibility of the
measures was around 0.2 µm. Biometric analysis of *H. karstenii* was conducted on 33 samples between 387 cm (~149 kyrs BP) and 818 cm (~355 kyrs BP) providing a mean temporal resolution of 6500 years (range of 1600 years to 16,000 years). Biometric analysis of *F. kerguelensis* was conducted on 10 samples between 3 cm (~2.7 kyrs) and 70 cm (~39 kyrs) providing a mean resolution of 4000 years (range of 2500 years to 7000 years) and on 33 samples between 387 cm (~149 kyrs BP) and 818 cm (~355 kyrs BP) providing a mean temporal resolution of 6500 years (range of 1600 years to 16,000 years) (Figs 3 and 4).

### 2.3. Sea-surface temperature reconstructions

Summer (January to March) SST was estimated by applying the Modern Analog Technique (MAT) to fossil diatom assemblages. The modern database is composed of 249 surface sediment samples, called analogs, for which locations modern summer SST were interpolated on a 1°x1° grid from the World Ocean Atlas (Locarnini et al., 2013) through Ocean Data View (Schlitzer, 2005). The MAT was implemented from the “bioindic” package (Guiot and de Vernal, 2011) developed for the R-platform. For each fossil sample, we used the relative abundances of 32 species and the chord distance to select the 5 most similar analogs. The threshold above which the selected analogs are supposed to be too dissimilar to the fossil assemblage is fixed at the first quartile of random distances on the validation modern dataset. Quantitative estimates of summer SST represent a distance-weighted average of the climate values associated with the selected and reliable modern analogs. This method yields a R² of 0.96 and a RMSEP of 0.96°C.

## 3. Results and discussion

The LAAD of *H. karstenii* at ~190 kyrs BP occurred when no other diatom species appeared or disappeared in the Southern Ocean (Barron, 2003). As such it is believed that *H. karstenii* did not *evolve into* a different lineage and that predation, *competition and/or taphonomic processes* did not play a major role. We here focus on the role of climate and environmental changes on the species survival with the main idea that *H. karstenii* could not acclimate to the harsh, high-stress conditions of the MIS 6.

### 3.1. Environmental conditions and *F. kerguelensis* size variations

We first focus on the last 40 kyrs to check that both environmental conditions and *F. kerguelensis* size variations recorded in core DCR-1PC follow what has been previously shown for the Subantarctic Zone (SAZ) (Cortese et al., 2012; Shukla et al., 2013; Nair et al., 2015, 2019). During this period, summer SST increased from ~4°C during the last glacial to around 8°C during the late Holocene (Fig. 4E), in line with previous studies from the SAZ (Becquey and Gersonde, 2002; Mazaud et al., 2002; Crosta et al., 2005b; Gersonde et al., 2005; Barrows et al., 2007; Martinez-Garcia et al., 2009; Benz et al., 2016; Nair et al., 2019). *Fragilariopsis kerguelensis* mean length congruently decreased from a peak of ~40 µm during the glacial period to a minimum of ~30 µm during the Holocene (Fig. 4A). The reduction in size was accompanied by a decrease in *F. kerguelensis* absolute abundances (Fig. 4B), suggesting a drop in the species productivity. By analogy to modern conditions, summer SST of ~4°C, *F. kerguelensis* mean length of ~40 µm and high *F. kerguelensis* abundances suggest that the Polar Front (PF) was located in the vicinity of the core site during the last glacial, with the frontal system migrating southward during the deglaciation. With inferred summer SST of ~8°C, *F. kerguelensis* mean length of ~30 µm and low *F. kerguelensis* abundances indicate that the SAF reached the core site during the early Holocene. Previous records from the SAZ similarly demonstrated a drop in *F. kerguelensis* mean length and in *F. kerguelensis* absolute abundances over the last deglaciation, which was attributed to a reduction in dust-borne iron supply to the Southern Ocean, a reduction in
nutrient supply in response to the southward migration of the frontal systems and associated
upwelling and an increase in summer SST limiting *F. kerguelensis* ability to uptake ambient silicate
and **biomineralize** its frustule (Cortese and Gersonde, 2007; Cortese et al., 2012; Shukla et al., 2013;
Shukla and Crosta, 2017; Nair et al., 2019).

Similar patterns in summer SST, *F. kerguelensis* mean length and absolute abundances are reconstructed during the previous glacial-interglacial cycles in core DCR-1PC. Interglacial periods were generally characterized by high summer SST, small and less abundant *F. kerguelensis*, but note that the MIS 7.4 stadial, centered at ~225 kyrs BP, was almost as cold as full glacial periods. This is in agreement with congruent low air temperatures as inferred from EPICA Dome C (EDC) deuterium (Fig. 4F) and large glacial ice volume as inferred from δ¹⁸O in benthic foraminifera (Lisiecki and Raymo, 2005). During this period, both *F. kerguelensis* mean length and absolute abundances increased from the very low values encountered during the warm MIS 7.5. Indeed, smallest mean length and abundances were found during warmest events of the MIS 7 and MIS 9, as well as during the early Holocene. Conversely, largest and most abundant *F. kerguelensis* were encountered during the MIS 6. During this period, very large *within-sample* standard deviations were also calculated (Fig. 4A) indicating a very large range in individual frustule *sizes*. Such a large range suggests the presence of very long initial cells along with very small specimens resulting from many asexual divisions under bloom conditions (Fig. 3A-C). Of specific interest is that the previous glacial period did not show *F. kerguelensis* as large *or* as abundant as in the MIS 6 despite relatively comparable mean summer SST, mean air temperature and mean global ice volume (Table 1). *Other* SST reconstructions *suggest* that the MIS 8 was slightly warmer than the MIS 6 (Becquey et al., 2002; Pahnke and Zahn, 2005; Martinez-Garcia et al., 2009). *Overall, we find that* the physical environment of the Southern Ocean was not drastically different between the last four glacial periods nor between the last four interglacial periods. At this stage, it is difficult to explain why the response of *F. kerguelensis* was exacerbated during the MIS 6 *or*, conversely, tamed during the MIS 8.

3.2. *Environmental conditions and Hemidiscus karstenii size variations*

In core DCR-1PC, *H. karstenii* absolute abundances reached up to ~60.10⁶ valves/g during the warmest periods of the MIS 9 and MIS 7 (Fig. 4D, E), when the SAF was inferred to be located in the vicinity of the core site. During these periods, the relative proportion of *H. karstenii* amounted to ~60 % of the total diatom assemblages (data not shown), in phase with previous studies from SAZ (Burckle et al., 1978; Burckle and Burak, 1988; Fenner et al., 1991; Crosta et al., 2004). The new data thus *confirm* a Subantarctic habitat for *H. karstenii*. Absolute abundances decreased rapidly to less than 20.10⁶ valves/g (>20 %) synchronously *with a* summer SST drop during the subsequent glacial inceptions to reach very low absolute and relative abundances during the glacial periods. The species still accounted for ~10.10⁶ valves/g (~10 % of the total assemblage) during MIS 8 until ~260 kyrs BP. *Hemidiscus karstenii* was almost absent in the core *for ~10 kyrs*, until its resumption at the beginning of the MIS 7. This pattern *did not repeat* during the MIS 6, whereby very *low* abundances (2-3.10⁶ valves/g or <3 %) of *H. karstenii* were found in sediments younger than its LAAD. Though the absolute and relative abundances of *H. karstenii* were not different during the late MIS 8 than during the late MIS 6, this species failed to reappear during the MIS 5, as it did during previous interglacials, *marking its extinction*.

The new morphometric measurements show that *H. karstenii* valves were smallest (~40 µm) during the late glacial periods (Fig. 4C) when the species abundances were lowest (Fig. 4D) and both air and ocean temperatures were very low (Fig. 4E,F). *Hemidiscus karstenii* mean length increased to ~50 µm during the subsequent interglacial periods. However, *H. karstenii* got even *larger* (~60 µm), when its abundances were maximum, during the long and warm MIS 7.1 to MIS 7.3 period. Both abundances and morphometric data suggest that the species then *thrived* at optimal conditions. By
analogy to findings on *F. kerguelensis* (Assmy et al., 2006; Crosta, 2009; Cortese et al., 2012; Shukla et al., 2013), it is believed that these optimal environmental conditions allowed *H. karstenii* to build larger initial cells yielding to overall larger *H. karstenii* populations, despite higher productivity as suggested by higher abundances of this species. Subsequent to these optimal conditions *H. karstenii* disappeared in 10-15 kyrs coincident with the glacial inception. The comparison to the previous glacial period may here give some insights into why this happened. The MIS 8 and MIS 6 both lasted around ~60 kyrs and environmental conditions were not very different though global ice volume was slightly larger and Antarctic temperatures along with Subantarctic SST were slightly lower during the MIS 6 (Table 1). However, climate evolution of each glacial period appears quite different. Indeed, the first half of the MIS 8 still experienced low global ice volume (Liesiecki and Raymo, 2005) congruent to high Antarctic and Southern Ocean temperatures (Becquey and Gersonde, 2002; EPICA Community Members, 2004; Pahnke and Zahn, 2005; Martinez-Garcia et al., 2009) and reduced sea ice cover (Wolff et al., 2006; Schneider-Mor et al., 2008). Overall, mild conditions still prevailed in the Southern Ocean during the first ~30 kyrs of the MIS 8 and glacial conditions developed only since ~280 kyrs BP. Such mild conditions may explain why *H. karstenii* was still abundant (~10.10⁶ v/g), with mean length of ~50 µm comparable to the previous interglacial, during the first half of the MIS 8. Conversely, the early MIS 6 experienced several millennial events of very cold climate, with the most dramatic one lasting between ~190 kyrs BP and ~181 kyrs BP (Fig. 4F), when *H. karstenii* LAAD occurred. During this period, glacial ice volume, air and ocean temperatures in the Southern Ocean along with sea ice reached full glacial conditions. High abundances as well as large sized *F. kerguelensis* (Fig. 4A,B) support that the APF was in the vicinity of the core site and that surface waters were rich in micro- and macro-nutrients (Petit et al., 1999; Schneider-Mor et al., 2008; Sigman et al., 2010; Jaccard et al., 2013). We therefore believe that iron and macro-nutrients per se cannot explain *H. karstenii* LAAD. As total diatom abundances were highest during this period (data not shown) and as diatom preservation is best around the APF (Burckle and Cirilli, 1987; Pondaven et al., 2000), it is believed that taphonomic processes cannot explain *H. karstenii* LAAD either.

Our new results show that SAZ conditions were rapidly replaced by APF conditions in DRC-1PC region across the MIS 7 to MIS 6 inception as a result of the congruent northern migration of the Southern Ocean hydrographic fronts, possibly pushing *H. karstenii* habitat northward. Unfortunately, the limited number of studies reporting *H. karstenii* (Fig. 1) does not allow tracking the glacial biogeography of this species. Of specific interest is whether its habitat could expand until South Africa if the Subtropical Front moved north (Howard and Prell, 1992; Bard and Rickaby, 2009; Romero et al., 2015) or was conversely much narrower if the Subtropical Front was constrained to its modern regional location due to a stronger Agulhas Return Current (Hays et al., 1976; Caley et al., 2014). For *H. karstenii* populations in DCR-1PC region, our results suggest that the fast changing ocean conditions across the MIS 7 to MIS 6 inception made the species more vulnerable or pushed it to its long-term adaptive limit, which prevented it from efficiently adapting, especially as this environmental degradation happened from a period (MIS 7.3 to MIS 7.1) very favorable to *H. karstenii*. The repetition of very cold events until the mid-MIS 6 may have further prevented *H. karstenii* from regrowing abundant communities, until its complete disappearance from core DCR-1PC during the late MIS 6.

It is difficult to assess from this single study what could have been the physiological drivers of *H. karstenii* size reduction and subsequent disappearance in core DCR-1PC. Based on our knowledge on *F. kerguelensis* we here suggest, as a working hypothesis, that the low SST prevailing during the early MIS 6 have reduced the ability of *H. karstenii* to efficiently uptake silicate and grow large initial cells. Dwarfism generally represents a survival strategy to high-stress environments by reducing the nutrient requirements (Keller et al., 2009; Wade and Olsson, 2009; Wade and Twichett, 2009). However, as the surface-to-volume ratio decreased along with mean length and width (S/V of 0.33
for a specimen of 60 µM * 43µm *8 µm during the MIS 7 and S/V of 0.125 for a specimen of 42 µm *
28 µm * 8 µm during the early MIS 6), a threshold might have been crossed when nutrient
transporter sites and uptake rates could not match the diatom intracellular nutrient requirements
(Hildebrand, 2000; Martin-Jézéquel et al., 2000; Marchetti et al., 2009). Additionally, the within-
sample large standard deviations on the mean lengths observed during the MIS 6 and resulting from
small maximum lengths and very small minimum lengths (Fig. 4C) suggest that many specimens
passed below the range of the sexually inducible size window and the few specimens that could
reproduce sexually formed small initial cells.

However, *H. karstenii* appears to have persisted at a few percent of the assemblage in MIS 6 sediments younger than its LAAD, in core DCR-1PC as well as other Southern Ocean cores. Thus, the precise timing of this species' true extinction, and the distribution of MIS 6 refuge areas (Burckle et al., 1978) remain open. If this species did not become extinct during MIS 6, the reasons why it did not repopulate during MIS 5, as it did during the previous interglacial periods remain unknown, though our first study on *H. karstenii* provides some options. The current interpretation of *H. karstenii* LAAD at ~0.19 Ma may reflect the functional extinction of this species, with trace occurrences in younger sediments resulting from reworking of this morphologically robust diatom. However, more studies dedicated to *H. karstenii* may shed light on the paleo-ecology and paleo-physiology of this taxon.

**4. Conclusion**

We here presented the first morphometric study of *H. karstenii*. Our *preliminary* study demonstrates that the species grew larger frustules during the interglacial periods and smaller frustules during the glacial periods over the MIS 10 to MIS 6. *Hemidiscus karstenii* also achieved greater abundances during the interglacial than during the glacial periods. However, larger frustules and higher productivity were found during the MIS 8 than during the MIS 6, which we related to differing climatic patterns during these glacial periods. The more rapid glacial inception and the repetition of very cold millennial-scale events during the early MIS 6 are thought to have committed *H. karstenii* to reduce its mean size and productivity. Conversely, the milder conditions during the first half of the MIS 8 may have allowed *H. karstenii* to preserve the same size as during the MIS 9. Based on these findings, we propose that *H. karstenii* did not have a sufficient ecological range to cope with the fast changing and repetitive harsh glacial conditions over the long MIS 6, preventing it from sustaining physiological viability, leading to its total disappearance and eventual extinction at the end of the MIS 6. However, this first study does not fully explain how *H. karstenii* survived previous glacial periods such as the MIS 12 and MIS 16, when climate conditions developed similarly to the MIS 6. More morphometric and quantitative studies should be conducted on *H. karstenii* in different settings and on longer timescales to better understand how this species adapted to older glacial periods and revived in subsequent interglacials.

**Acknowledgments**

We are grateful to chief scientist Dr Y. Nogi (National Institute of Polar Research), and the captain,
crew, scientists and students of Cruise KH10-07 onboard R/V Hakuho-Maru in 2010. We are thankful
to two anonymous reviewers, guest editor Reed Scherer and Marine Micropaleontology editor-in-
chief Ric Jordan for their helpful comments and corrections. This work was funded by the SERB
Overseas Postdoctoral Fellowship 2017 attributed to SKS to visit XC and the JSPS KAKENHI (Grants-in-
Aid for Scientific Research) Grant Numbers JP19340156, JP23244102, JP17H06318 attributed to MI.
## Data availability

Summer sea-surface temperatures, *Fragilariopsis kerguelensis* and *Hemidiscus karstenii* data from core DCR-1PC presented in this manuscript are publicly available at https://doi.pangaea.de/10.1594/PANGAEA.913534.

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**Table caption**

**Table 1**

Mean values of $\delta^{18}$O$_{benthic}$ (representative of global ice volume), deuterium in EDC (representative of Antarctic air temperatures) and summer sea-surface temperatures in the southwestern Indian sector of the Southern Ocean during the Marine Isotope Stages covered by the biometric analyses conducted here.
## Figure captions

**Fig. 1.** Location map of core DCR-1PC (star) and cores with mention of *Hemidiscus karstenii* during the Marine Isotope Stage 7 (filled circle, with the color reflecting the relative abundances of the species) or previous geological periods (white squares). The list of studies is presented in the Supplementary Table 1. The background map represents the annual mean sea-surface temperature field derived from the World Ocean Atlas 2013 (Locarnini et al., 2013). Position of the fronts according to Orsi et al., (1995). 1: Burckle et al., 1978; 2: Baldauf and Barron, 1991; 3: Ciesielski, 1983; 4: Crosta et al., 2004; 5: Fenner, 1991; 6: Gersonde and Bárcena, 1998; 7: Graves, 1982; 8: Zielinski and Gersonde, 2002; 9: Harwood and Matuyama, 1992; 10: Burckle and Burak, 1988; 11: Giulani et al., 2011; 12: Konfirst et al., 2012; 13: Pudsey et al., 2000; 14: Winter and Iwai, 2002; 15: Tolotti et al., 2018; 16: Lee et al., 2012; 17: Nees et al., 1999. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article)

**Fig. 2.** DCR-1PC core chronology. A: Final age tie-points used to determine the age model; B: comparison of DCR-1PC diatom-based SST record (red curve) on core depth to EDC deuterium record (black curve) on ice core age along with few tie-points; C: comparison of the SST and deuterium records vs age. The gray bands represent the warm marine isotopic stages. LAAD Hk: Last Abundant Appearance Datum of *Hemidiscus karstenii*. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article)

**Fig. 3.** Phase contrast images of *Fragilariopsis kerguelensis* (A-C) and *Hemidiscus karstenii* (D-F) showing the range in size and morphologies encountered for both species in core DCR-1PC, along with the measurements performed on each specimen. L: Length; W: Width; Fk: *Fragilariopsis kerguelensis*; Hk: *Hemidiscus karstenii*; Hc: *Hemidiscus cuneiformis*. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article)

**Fig. 4.** DCR-1PC records of *Fragilariopsis kerguelensis* and *Hemidiscus karstenii* mean length (A and C, respectively) and absolute abundances (B and D, respectively), and summer SST (E) along with EDC record of deuterium (F). The vertical whiskers represent within-sample standard deviations, i.e. the standard deviation on the 100 length measurements performed on each species in each sample. The gray bands represent the warm isotopic stages. LAAD Hk: Last Abundant Appearance Datum of *H. karstenii*. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article)
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A

<table><thead><tr><th>Mid-depth (cm)</th><th>Species</th><th><sup>14</sup>C age (yr)</th><th>Reservoir age (Bard1988) (yr)</th><th>Reservoir age corrected 14C age (yr)</th><th>Calibrated Age (1 sigma) (yr BP)</th><th>Calibrated Age (yr BP)</th></tr></thead><tbody><tr><td>0.58</td><td>G. bulloides</td><td>2750 ± 30</td><td>890</td><td>1860</td><td>1821</td><td>1907</td><td>1864</td></tr><tr><td>14.45</td><td>G. bulloides</td><td>7115 ± 40</td><td>890</td><td>6225</td><td>7119</td><td>7227</td><td>7173</td></tr><tr><td>24.75</td><td>G. bulloides</td><td>12025 ± 50</td><td>890</td><td>11135</td><td>12941</td><td>13096</td><td>13019</td></tr><tr><td>32.70</td><td>G. bulloides</td><td>14085 ± 60</td><td>890</td><td>13195</td><td>15745</td><td>15960</td><td>15853</td></tr><tr><td>40.66</td><td>N. pachyderma</td><td>22060 ± 105</td><td>890</td><td>21170</td><td>25382</td><td>25652</td><td>25517</td></tr><tr><td>59.98</td><td>N. pachyderma</td><td>32770 ± 200</td><td>890</td><td>31880</td><td>35554</td><td>36036</td><td>35795</td></tr><tr><td>80.43</td><td>N. pachyderma</td><td>40480 ± 395</td><td>890</td><td>39590</td><td>42915</td><td>43601</td><td>43258</td></tr><tr><td>116.8</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>59330</td></tr><tr><td>155.4</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>70230</td></tr><tr><td>163.4</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>72030</td></tr><tr><td>182.7</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>75830</td></tr><tr><td>197.5</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>84030</td></tr><tr><td>237.1</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>95030</td></tr><tr><td>250.6</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>101230</td></tr><tr><td>265.2</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>105730</td></tr><tr><td>290</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>119530</td></tr><tr><td>313.7</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>125530</td></tr><tr><td>340.9</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>130130</td></tr><tr><td>354.6</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>136130</td></tr><tr><td>367.1</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>138530</td></tr><tr><td>456.8</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>186130</td></tr><tr><td>576.2</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>224630</td></tr><tr><td>637.4</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>253330</td></tr><tr><td>695.1</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>274130</td></tr><tr><td>755.4</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>316730</td></tr><tr><td>785.1</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>335930</td></tr><tr><td>820.5</td><td>Tuning to EDC d</td><td></td><td></td><td></td><td></td><td></td><td>356830</td></tr></tbody></table>

B

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C

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<figure><img src="image_11.png" /></figure>
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<table><thead><tr><th></th><th>MIS 1</th><th>MIS 2-3</th><th>MIS 6</th><th>MIS 7</th><th>MIS 8</th><th>MIS 10</th><th>MIS 12</th></tr></thead><tbody><tr><td>Temporal range LR04 (kyrs)</td><td>0-14</td><td>14-40</td><td>130-191</td><td>191-243</td><td>243-300</td><td>337-374</td><td>424-478</td></tr><tr><td>Mean δ<sup>18</sup>O<sub>benthic</sub> LR04 (‰)</td><td>3.4</td><td>4.7</td><td>4.6</td><td>3.8</td><td>4.3</td><td>4.5</td><td>4.6</td></tr><tr><td>Temporal range EDC (kyrs)</td><td>0-16</td><td>16-40</td><td>134-191</td><td>191-247</td><td>247-304</td><td>341-380</td><td>430-477</td></tr><tr><td>Mean δD EDC (‰)</td><td>-396.5</td><td>-435.6</td><td>-434.1</td><td>-413.4</td><td>-430.6</td><td>-433.5</td><td>-432.5</td></tr><tr><td>Mean SST DCR1PC (°C)</td><td>7.6</td><td>4.3</td><td>4.4</td><td>7</td><td>4.7</td><td>4.3</td><td></td></tr></tbody></table>