Material and site description

The CALYPSO core MD07-3088 was retrieved during the IMAGES PACHIDERME (MD 159) expedition by the French R/V Marion Dufresne off Southern Chile (46°04 S; 075°41 W), at a water depth of 1536 m, i.e., well above the modern lysocline (around 3700 m)55 (Fig. 1). The site is bathed by the upper layer of southward flowing Pacific Deep Water (PDW), at the boundary with northward flowing AAIW2. The surface waters above the site are on the direct northward path of the SSW that is fed by the DIC- and nutrient-rich surface waters of the Antarctic Zone. These Antarctic and Subantarctic surface waters represent the source for AAIW/SAMW (i.e., intermediate depth waters), and have been hypothesized to be a major conduit through which high-latitude ocean changes are transmitted to the lower latitudes2. Core MD07-3088 has the advantage of being located within the Subantarctic Zone, well to the south and well to the north of the Tropical and Polar Frontal Zones, respectively, and has probably not been affected by potential shift of the Subtropical Front (STF) or the Subantarctic Front (SAF) in the past. The southern tip of Chile is the only continental mass intercepting the westerly winds within this latitude range, generating a zone of high precipitations that result in high fluvial sediment supplies to the South Pacific Ocean. Since any significant sediment reworking is precluded at site MD07-308814, the extremely high sedimentation rates recorded at site MD07-3088 (∼300 cm/kyr during the Last Glacial and ∼60 cm/kyr during the deglaciation and the Holocene14) provide a rare opportunity to study productivity patterns of the Subantarctic Zone with decennial to centennial resolution during the last glacial termination.

Age models of core MD07-3088 and ODP cores 1238 and 1233

The MD07-3088 age model has been determined using SH1314 as the 14C Southern Hemisphere calibration curve57. In order to compare our micropaleontological and geochemical records with ODP sites 1233 and 1238 located in the South-Eastern Pacific (SEP, 41.0°S, 74.4°W) and the Eastern Equatorial Pacific (EEP, 1.5°S, 82.5°W) respectively, we established a common age model for these cores to test temporal phasing since the late glacial period. For ODP site 1233, we use the recently updated age model of ref. 58 and based on the reservoir 14C age estimates by ref. 14. The age model of ODP site 1238 is based on 10 AMS 14C dates obtained on planktonic foraminifera Neogloboquadrina dutertrei using a constant local sea-surface reservoir 14C age (R S ) correction (ΔR = 72 ± 35 yr) based on previous regional estimates15. However, this approach did not consider the possible advection of old subsurface waters in particular during the deglaciation as suggested by previous studies at local and regional scales59,60. Since no independent R S estimates are available in literature for the EEP, other methods must be considered in order to obtain a robust common stratigraphic framework. Hence, we first compared the planktonic foraminifera δ13C records of the two cores versus conventional 14C age14,15, the δ13C record for ODP 1238 representing sub-surface record as it is measured on N. dutertrei (Supplementary Fig. 1). In general, the first order (and most of the second order) changes in planktonic δ13C are recorded in both cores. These results also match co-existing benthic–planktonic foraminifera (B-P) 14C and δ13C differences in core MD07-3088, indicating variations in oceanic ventilation (see ref. 14 for extensive discussion) versus conventional 14C age. Similarly, the ventilation changes expressed in term of upwelling increases observed in core MD07-3088 were coeval with changes in surface ocean carbon content in the EEP15 (Supplementary Fig. 1). Through these comparisons, it is clear that enhanced mixing (between ~15 and ~13.1 14C ka, and between ~12 and ~10.5 14C ka) was characterized by a lower difference between planktonic and benthic carbon isotope signatures, and are globally synchronous with oceanic pCO 2 changes. This finding supports the hypothesis that the planktonic foraminiferal records correspond to the water masses with the same history (SAW and SAMW) presenting similar radiocarbon contents. This allows us to deduce that EEP and SEP were characterized by similar reservoir 14C age changes at least since the last deglaciation.

Coccolith slides and morphometric measurements (SYRACO)

Slides of 80 samples were prepared at GEOPS laboratory. Briefly, ~0.03 g of sediment was diluted in 28 mL Luchon water (pH = 8, bicarbonate = 78.1 mg per liter, total dissolved solid = 83 g per liter) within a flat beaker, and settled on a 12 × 12 mm coverslip for 4 h 30 min. The coverslip was then oven-dried at 70 °C, and mounted on slides with NOA74. This technique ensures a homogenous distribution of coccoliths and allows quantifying the amount of material per gram of sediment61 as follow:

$${A} = \left( {{\mathrm{Nc}}\,\times\,{\mathrm{Sf}}} \right)/\left( {\mathrm{No}}\,\times\,{\mathrm{So}\,\times\,{\mathrm{Ws}}} \right)$$ (1)

where A is the number of coccoliths per gram of sediment; Nc is the number of counted coccoliths (between 505 and 3900); Sf is the surface of the flat beaker (3117 mm2) in which suspended sediments (and coccoliths) settle; No is the number of view fields (165); So is the surface of a view fields (0.01 mm2) and Ws is the weight of sediment that settled in the flat beaker (between 0.018 and 0.043 mg).

For each sample, abundance and morphometric analyses (length, width, area, mass) of individual coccoliths were automatically obtained with an average of 1591 coccoliths per sample, by the SYRACO software using automated microscope (Leica DM6000B). SYRACO performs pattern recognition under cross-polarized light using artificial neural networks61. It detects and classifies most of the coccoliths present in the samples throughout the time series (mainly represented by Emiliania huxleyi, Gephyrocapsa muellerae, Gephyrocapsa oceanica, Calcidiscus leptoporus, and Helicosphaera carteri). Coccolith mass were directly deduced based on a quasi-linear relationship that exists between their brightness (birefringence in grey scale colors) and their thickness under cross-polarized light. Because this method applies on coccoliths thinner than 1.55 μm that exhibit grey scale colors61, we interpret only thickness and mass measurements for Noëlaerhabdaceae coccoliths. Indeed, their abundance and morphometric parameters show standard error of ±1% and ±3% in each sample respectively. Morphometric analyses for C. leptoporus and H. carteri that display third-order interference colors (and thus increasing standard errors), are only presented within the Supplementary Information (Supplementary Fig. 2).

Since coccolith mass are not independent of coccolith size, we calculated size-normalized thickness indices for all the Noëlaerhabdaceae coccoliths within each sample to verify that changes in coccolith mass represent changes in calcification, according to the two equations that exist so far27,28,44 (Fig. 3). We obtained the Size Normalized Thickness index SN27,44 that considers coccolith thickness related to cell surface area as follow:

$${\mathrm{SN}}_{{\mathrm{thickness}}}\,(\upmu {\mathrm {m}}) = \, \left[ {\left( {\mathrm{ML}}\,-\,{\mathrm{CL}} \right) \times {S}} \right]\,+\,{\mathrm{CT}}$$ (2)

Where ML is the mean coccolith length over the whole time serie, CL is the length of coccolith X in Sample A, S is the slope of the linear regression between coccolith length and coccolith thickness for all coccolith in Sample A, and CT is the original thickness of coccolith X in Sample A (i.e., coccolith mass/coccolith area ratio).

We calculated the lateral cross-sectional aspect ratio AR L 28 that considers coccolith thickness related to cell volume as follow:

$${\mathrm{AR}}_{\mathrm{L}}\left( {{\mathrm{dimensionless}}} \right)={T}_{\mathrm{L}}{\mathrm{/}}\surd {{A}}_{\mathrm{L}}$$ (3)

where T L and A L are the thickness and the area of coccolith X in Sample A respectively. T L = M L /A L , i.e. coccolith mass (M L )/coccolith area (A L ).

Coccolith area and mass values as well as SN thickness and AR L values obtained herein are in the same order of magnitude than published data using similar birefringence-based methods27,28,39.

Coccolith taxonomy and preservation

More than 96% of the assemblages were composed of five species: Emiliania huxleyi, Gephyrocapsa muellerae, Gephyrocapsa oceanica, Calcidiscus leptoporus, and Helicosphaera carteri. As for modern settings, Emiliania and Gephyrocapsa, that constitute the Noëlaerhabdaceae family, represent the most prominent genera (from 81 to 97% of the assemblages) and reflect the main patterns of the total coccoliths. For that reason, but also because smallest Emiliania and Gephyrocapsa from the SE Pacific present a wide range of morphotypes62 that are not easily classified under light microscope, we mainly considered the Noëlaerhabdaceae family instead of Emiliania and Gephyrocapsa species. Besides, species assignations within the Noëlaerhabdaceae family are primarily based on size43, and all narrowly restricted size classes of Noëlaerhabdaceae present the same main patterns (Supplementary Fig. 2). Indeed, generally, <3 μm Noëlaerhabdaceae represent E. huxleyi type C and small Gephyrocapsa; 3–4 µm Noëlaerhabdaceae are associated to E. hyxleyi type B/C and G. muellerae; and >4 µm Noëlaerhabdaceae document E. huxleyi type A and B and G. oceanica patterns43,62.

This study gathers specific morphological parameters of exactly 152,809 coccoliths that appear to reflect primary biomineralization features. The core MD07-3088 has been retrieved well above the lysocline. It is mainly made of homogenous fine-grained material that, together with high sedimentation rates (∼300 cm/kyr during the Last Glacial and ∼60 cm/kyr during the deglaciation and the Holocene14), prevent post-depositional fluid circulations. Besides, dissolution processes trigger a strong differential preservation of coccoliths keeping resistant specimens and losing delicate ones. The most delicate morphotypes belong to the Noëlaerhabdaceae family that represent the main coccolith of the assemblage. The smallest Noëlaerhabdaceae (<3 μm, i.e., mainly E. huxleyi type C and small Gephyrocapsa) depict the same exact pattern as the larger ones, with higher masses when the oceanic carbon reservoir is reconnected to the surface waters and bring CO 2 -rich waters into the photic zone (Supplementary Fig. 2), while such conditions could have favored the dissolution of coccoliths in the water column. At last, the three main increases observed during the deglaciation in the mean Noëlaerhabdaceae coccolith mass, would not be biased by diagenetic overgrowth that would also affect C. leptoporus and to a lesser degree H. carteri, that generally depict however, reducing coccolith masses during these time intervals (Supplementary Fig. 2). Indeed, diagenetic processes (dissolution or overgrowth) would simultaneously impact all coccolith morphotypes, without any discrimination between morphotypes.

Foraminifera abundance and mass data

Planktonic foraminifera assemblages were determined at the LSCE (Laboratoire des Sciences du Climat et de l’Environnement) counting at least 300 specimens per sample. From three different depths (570, 950, and 990 cm), we weighted 30 individuals from the most abundant species (Neogloboquadrina pachyderma (sinistral and dextral coilings), Globigerina bulloides, Globorotalia inflata, Turborotalita quinqueloba and Globigerinita glutinata) for different sizes (>450 µm, 315–450 µm, 250–315 µm, 150–250 µm) to determine their mean weight. For this core, we obtained mean weights of: 7 ± 2 µg for N. pachyderma, T. quinqueloba, and G. glutinata, 18 ± 3 µg for G. bulloides, and 19 ± 5 µg for G. inflata. For Globigerinella calida, Globigerina falconensis, G. ruber, Globigerina hexagonus (representing <2% in all samples), Neogloboquadrina dutertrei and Hastigerina digitata (representing <0.5% in all samples), we assumed a mean weight similar to G. bulloides. At last, for Globorotalia truncatulinoides, Globorotalia crassaformis, and Globorotalia hirsuta, we assumed a mean weight similar to G. inflata. From the assemblage and the mean weight of the different species, we estimated the planktonic foraminifera calcite mass for each sample, CaCO 3pl. foram. mass in mg/g as follow:

$${\mathrm{CaCO}}_{3{\mathrm{pl}}.{\mathrm{foram}}.{\mathrm{mass}}} = \frac{{{N} \times 2^{{\mathrm{split}}}}}{M} \times \mathop {\sum }

olimits_{i} \left( {m_{i} \times X_{i}} \right)$$ (4)

where N is the total amount of determined foraminifera (≥300), split is the number of split done before establishing a planktonic assemblage, M is the total dry mass of the sample (g), m i the mean weight of the species i (mg), and X i the percentage of the species in the sample.

This approach is a first order estimate of the foraminifera mass percentage as it does not fully take into account smaller species often <150 µm (such as G. uvula and partly T. quinqueloba) and juveniles. Besides, for 16 depths (covering LGM, HS1, ACR, YD and the Holocene), we weighted 6 to 60 specimens of G. bulloides (the most abundant foraminifera) from different size ranges (150–200, 200–250, 250–315, 315–355, 355–400, and 400–450 µm) in order to statistically characterize potential weight changes within a narrow size range. Mean weights for the different size classes decrease of about 20% from LGM to Holocene, and of about 7 and 18% during HS1 and YD respectively. If similar weight decreases are observed within the other planktonic species, the magnitude of the changes in the overall weight (∼20%) would be not sufficient enough to significantly change the estimated planktonic foraminifera mass flux. Indeed, because of the drastic increases within the planktonic foraminifera abundance during these time intervals (more than one order of magnitude), fluctuations in the planktonic foraminifera weights would imply changes in the flux of planktonic foraminifera calcite mass that remain within the error bars.

Total CaCO 3 and organic carbon analyses

Total CaCO 3 was determined at GEOPS laboratory using the vacuum-gasometric technique with a precision better than ±2%. 100 mg (±5) of crushed-dried sediments react with a few milliliters of HCl 6 N in a hermetic reaction chamber (22.4 cm3) that is connected to a manometer MANO MEX2-420 that measures the amount of outgassed CO 2 . The system is calibrated so that 100 mg of CaCO 3 (100%) trigger a pressure rise to 1 bar.

Total organic carbon and nitrogen contents together with organic matter δ13C analyses were obtained at the LSCE, using an Elementary Analyzer (Flash EA 1112) and the online continuous EA coupled with an Isotopic Ratio Mass Spectrometer (Finigan Delta + XP). The results are expressed in % C, % N, and in δ13C per mL (‰) against the international standard V-PDB (Vienna Pee Dee Belemnite). Error margin is defined according to the source linearity checked for each run based on internal home-standard (ΔC < 0.03% and Δδ13C < 0.2‰). A aliquot of <250 μm of dry sediment is softly leached with ultra-pure HCl 6 N to remove carbonate and dry at 50 °C. The samples were then crushed in a pre-combusted glass mortar for homogenization prior to carbon, nitrogen content and δ13C analyses.

XRF scanner measurements

The high-resolution elemental analysis of Br and Ca was performed using an Avaatech profiling X-ray fluorescence (XRF) core scanner at Royal Netherland Institute for Sea Research (NIOZ) at a 1 cm downcore resolution. The external reproducibility of this core-scanner for Br and Ca in the range of the measurements is below 2% (1σ).

Sedimentary POC: PIC ratio vs POC: PIC rain ratio (1/ρ)

It remains difficult to evaluate the influence of changes in the TOC relative to the CaCO 3 (POC/PIC ratio, 1/ρ) water column export and sedimentary burial on past pCO 2 variability. Indeed, the amount of particulate organic and inorganic carbon in the sediments is not necessarily directly related to the fraction exported from the surface waters. While it is probably reasonable to assume that the CaCO 3 accumulated in the sediment is representative of the PIC exported from the mixed layer to deep waters as core MD07-3088 was retrieved well above the lysocline (located around 3700 m depth nowadays55), it is probably not the case for TOC that might be more easily mineralized within the water column and upper sediments. However, at site MD07-3088, the combination of high sedimentary TOC contents (up to 1.9%), high sedimentation rates, and homogeneous fine-grained lithology, lead us to assume that post-depositional remineralization processes associated to (O 2 -rich) fluid circulations within the sediments must be of secondary importance. Moreover, it has been shown that it is in fact the oxygen exposure time that determines organic carbon degradation (i.e., ref. 63), and based on the considerations above, we infer that labile organic compounds must have been buried rapidly, minimizing the potential for selective alteration. There is no doubt that remineralization processes that occurred within the water column (and particularly the twilight zone), altered the downward flux of POC, and thus the efficiency of carbon sequestration. However, the latitudinal distribution pattern of POC in surface sediments along the Chilean margin55 reflects satellite-derived surface-ocean chlorophyll concentrations64, which indicates that sedimentary TOC concentrations primarily reflect OC export rather than selective degradation processes within the water column. Besides, the high-latitude, iron-fertilized, near-shore ecosystem that characterize site MD07-3088, seems to be the perfect candidate to promote the sinking of organic matter to the deep seafloor65,66,67. Therefore, in order to consider a wide range of POC transfer efficiencies65,66,67, we have tested the impact of BCP for HS1 and ACR, in cases where 10 to 50% of the exported POC is preserved within the sediments. Figure 5 indicates the influence of 1/ρ on seawater carbonate chemistry for cases ranging from photosynthetic processes to calcification processes only (solid black arrows) and for 10% to 50% of the TOC exported flux preserved in core MD07-3088 sediments for the HS1 and ACR periods.

Data availability

The data that support the findings of this study are available from the corresponding author (S.D.-A.) upon reasonable request.