# Increased El Niño amplitude during the last deglacial warming

Marco Yseki, Bruno Turcq, Dimitri Gutierrez, Renato Salvatteci, Dante Espinoza Morriberón, Hugues Boucher, Philippe Martinez, Matthieu Carré

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Marco Yseki, Bruno Turcq, Dimitri Gutierrez, Renato Salvatteci, Dante Espinoza Morriberón, et al. Increased El Niño amplitude during the last deglacial warming. 2022. hal-03813693

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# Increased El Niño amplitude during the last deglacial warming

**Marco Yseki** (✉ marco.yseki@gmail.com)

LOCEAN-IPSL Laboratory (UMR7159 CNRS-IRD-MNHN-Sorbonne Universit&#x0019;es)

https://orcid.org/0000-0002-8024-0615

**Bruno Turcq**

Center IRD FranceNord, IRDSSorbonne University (UPMC, Univ. Paris 06) CNRS/MNHN, LOCEAN
Laboratory, Bondy

**Dimitri Gutierrez**

Instituto del Mar del Perú (IMARPE)

**Renato Salvatteci**

Kiel University. Center for Ocean and Society

**Dante Espinoza-Morriberón**

Instituto del Mar del Perú

**Hugues Boucher**

LOCEAN-IPSL Laboratory (UMR7159 CNRS-IRD-MNHN-Sorbonne Universit&#x0019;es)

**Philippe Martinez**

EPOC

**Matthieu Carré**

LOCEAN-IPSL Laboratory (UMR7159 CNRS-IRD-MNHN-Sorbonne Universit&#x0019;es)

## Article

### Keywords:

**Posted Date:** February 17th, 2022

**DOI:** https://doi.org/10.21203/rs.3.rs-1303299/v1

**License:** cc license This work is licensed under a Creative Commons Attribution 4.0 International License.

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Increased El Niño amplitude during the last deglacial warming

Marco Yseki¹*, Bruno Turcq¹, Dimitri Gutiérrez²,³, Renato Salvatteci⁴, Dante Espinoza-Morriberón⁵,⁶, Hugues Boucher¹, Philippe Martinez⁷ & Matthieu Carré¹,⁸

1 LOCEAN-IPSL, Laboratoire d'Océanographie et du Climat: Expérimentation et Approches Numériques, Sorbonne Université/CNRS/IRD/MNHN, Paris, France.

2 Dirección General de Investigaciones Oceanográficas y de Cambio Climático, Instituto del Mar del Peru, Callao, Peru.

3 Laboratorio de Ciencias del Mar, Facultad de Ciencias y Filosofía, Universidad Peruana Cayetano Heredia, Lima, Peru.

4 Center for Ocean and Society, Kiel University, Kiel, Germany

5 Instituto del Mar del Peru, Laboratorio de Modelado Oceanográfico y de Cambio Climático, Callao, Peru.

6 Universidad Tecnológica del Peru (UTP), Facultad de Ingeniería, Lima, Peru.

7 UMR 5805 EPOC, Université de Bordeaux-CNRS-EPHE. Pessac Cedex, France.

8 Universidad Peruana Cayetano Heredia, Facultad de Ciencias y Filosofía, Centro de Investigación para el Desarrollo Integral y Sostenible, Laboratorios de Investigación y Desarrollo, Lima, Peru.
Abstract

It is still unclear how El Niño Southern Oscillation (ENSO), the leading mode of global-
scale interannual climate variability, will respond to global warming. The last
deglaciation offers natural experimental conditions to observe the behavior of ENSO in
a period of abrupt warming and sea level rise. Here we present a record of ENSO-related
interannual variability of river discharge in Peru during the last deglaciation (17.5-13 kyr
BP) and the Late Holocene (2.7-1.4 kyr BP), based on high resolution records of Titanium
in marine sediments from the Peruvian margin (Callao, 12°S and Pisco 14°S). We find
that the amplitude of ENSO events was 50 to 190 % larger during the deglaciation
compared to the Late Holocene, which supports the hypothesis that ENSO in the Eastern
Pacific is strengthened by ice sheet meltwater discharge. A possible strengthening of
ENSO in response to future ice sheet melting should be considered.
## Introduction

The El Niño-Southern Oscillation (ENSO) is the main mode of interannual climate variability in the world¹,² and especially affects South American precipitation through atmospheric teleconnections³,⁴,⁵,⁶. Large uncertainties in projections of ENSO reflect important gaps in our understanding of ENSO natural variability and its relationship with the mean climate state⁷,⁸,⁹. Reconstructions of ENSO centennial to millennial scale response to past natural climate change provide insights into the sensitivity of ENSO to changes in the background state and observational constraints to climate models¹⁰,¹¹.

The last deglaciation (~19-11 kyr BP) was a period of rapid global warming associated with a ~80 ppm increases in atmospheric CO₂¹². The deglacial climate was also characterized by the melting of polar ice sheets¹³ causing a rapid sea level rise¹⁴ and a series of abrupt millennial-scale events¹⁵. The behavior of ENSO during the last deglaciation is still unclear. In the Eastern Equatorial Pacific (EEP), the spread of individual foraminifera Mg/Ca and δ¹⁸O values estimated in three deglacial snapshots suggest a 16 to 60% increase in ENSO variability at 12.5, 15.1 and 17.9 kyr BP compared to the Late Holocene (1.6 kyr BP)¹⁶, while no increase was found at 16.8 kyr BP using a different foraminifera species¹⁷. The lithic flux measured in the 106KL marine core from Callao, one of the few high-resolution records in the Eastern Tropical Pacific (ETP), was interpreted as a proxy for El Niño intensity¹⁸. However, the lithic flux has a fluvial and aeolian mixed origin in Callao¹⁹, and the record's resolution for the last deglaciation did not allow to resolve the ENSO frequency band in this core.

In paleoclimate model experiments, deglacial meltwater pulses in the North Atlantic induce a weakening of the Atlantic Meridional Overturing Circulation (AMOC) intensity, a southern shift of the Intertropical Convergence Zone (ITCZ) and an increase in ENSO variability²⁰,²¹,²²,²³. Meltwater forcing has been invoked to explain that ENSO in the early
Holocene was not as reduced as predicted by insolation forcing alone²⁴,²⁵, but still requires to be tested with records from the deglaciation, when the meltwater flux was at its highest level. The last deglaciation is not equivalent to the current anthropogenic global warming, but offers us the opportunity to assess how ENSO characteristics were affected by this abrupt climate change.

# Results and discussion

## A high-resolution sediment record of ENSO variability

We present here a record of fluvial discharge interannual variability derived from high-resolution X-ray fluorescence Titanium (Ti-XRF) counts in two marine sediment cores collected on the Peruvian margin (Fig. 1). The core M77/2-005-3 (12°05 S, 77°40,07 W) was collected off Callao at 214 m depth, and core G14 (14.38°S, 76.42°W) was collected off Pisco at 390 m depth. Despite sedimentation hiatus due to erosion, both cores record the last deglaciation (17.5-13 kyr BP) and a Late Holocene section (2.7-1.3 kyr BP) is recorded in Callao¹⁹ (see depth-age models in supplementary Fig. 1). The sedimentation rate in the deglacial period, determined by 19 radiocarbon dates in Callao and 17 in Pisco is stable and high over the period (supplementary Fig. 1). The sections analyzed show irregularly spaced but well-preserved laminae indicating minimized sediment mixing (supplementary Fig. 2 and 3). The fast sedimentation (Late Holocene: 0.5 mm/yr in Callao; deglaciation: 1.2 mm/yr in Callao and 0.7 mm/yr in Pisco)¹⁹ combined to the high resolution of the XRF measurements (1 mm) yields an unprecedented record of interannual climate variability in the ETP over thousands of years.
<figure><img src="image_2.png" /><figcaption>Figure 1. A) Location of the sampling of the sediment cores M77/2-005-3 (black circle) and G14 (black triangle) in the Peruvian margin. B) Linear regression coefficients between ENSO Eastern Pacific indices and precipitation (Figure adapted from Sulca *et al.*⁵).</figcaption></figure>

Since Ti is relatively immobile during chemical weathering, Ti has been widely used as an indicator of terrigenous detrital input to sediments²⁶. Ti-XRF variations in the central-southern Peruvian margin has been shown to be related to the fluvial fraction of terrestrial input¹⁹ and is therefore used here as a proxy for river discharge (see Methods). The core M77/2-005-3 off Callao receives a Ti flux primarily from the discharge of the rivers Chancay, Chillón, Rimac, and Lurín, while the G14 core off Pisco is influenced by the Pisco and Ica rivers. Under the current climate, the flow of coastal rivers is usually low and increases during austral summer because of the monsoon rainfall in the High Andes³,²⁷,²⁸. During El Niño events, especially those with high sea surface temperature (SST) anomalies in the Eastern Pacific (EP), strong thunderstorms and precipitation occur on the coast⁵,²⁹,³⁰,³¹,³². These events produce catastrophic flash floods and large discharge of sediments during a few days³³,³⁴,³⁵.
Since Ti is related to river discharge and ENSO is the primary driver of interannual variability of rainfall in Peru, on the coast as well as in the Andes⁵, the 2.5-8 year frequency band of Ti-XRF in Pisco and Callao sediment cores provide us with an unprecedented 4000-year long record of ENSO-related rainfall interannual variability during the last deglaciation. Ti-XRF counts records were normalized so that both cores could be compared (supplementary Fig. 4). Changes in the amplitude of interannual variability were estimated by the standard deviation of the 2.5-8 years band-pass filtered Ti-XRF series calculated over a 300-years long sliding window (see Methods).

## Precipitation and ENSO in Peru during the deglaciation

Strong similarity is observed in the variations of Ti-XRF in Callao and Pisco during the last deglaciation, considering the chronological uncertainty (Fig. 2a). More importantly, the amplitude of Ti-XRF interannual variability varies coherently on millennial timescales in both sites (Fig. 2c). The regional scale reproducibility of the records demonstrates that the dataset is not affected by analytic or local biases and that Ti-XRF variations faithfully reflect regional scale climate variability.

Compared to the 1400-year long Late Holocene section, which can be considered as representative of modern conditions based on multiple studies¹⁶,¹⁷, ²⁴,²⁵,³⁶,³⁷,³⁸, higher Ti-XRF is found during the deglacial period (Fig. 2a). Lower sea level during the deglaciation might be partly responsible for the higher Ti-XRF off Callao and Pisco because of the reduced distance to the river mouths. However, the sediment source was only 20 km closer to the Callao core 18 kyr BP compared to today and at about the same distance for the Pisco core. In addition, the sea level rose strongly from 18 to 13 kyr BP, a trend that is not reflected in the Ti-XRF record, neither in Callao nor in Pisco. Hence, the larger deglacial Ti-XRF is likely the result of increased river discharge in central-
southern Peru. A deglacial increase in sediment discharge with a marked maximum
during the Heinrich Stadial 1 cold spell was also recorded in a marine core from
northern³⁸ and central-southern Peru¹⁹. Since coastal precipitation only occasionally
occurs during strong El Niño events, the larger average river flow was likely primarily
caused by increased precipitation in the Andes in agreement with records of strengthened
south American monsoon in the central Andes³⁹,⁴⁰,⁴¹,⁴² and evidence of a southern shift
of the ITCZ mean position at that time⁴³,⁴⁴,⁴⁵.

We find a 50 to 190% increase in the amplitude of ENSO frequency band variability
during the deglacial climate (17.5-13 kyr BP) compared to the Late Holocene (2.7-1.4
kyr BP) (Fig. 2c and 3g), when ENSO had similar to modern characteristics¹¹,¹⁷,²⁴,²⁵,⁴⁶.

The record shows centennial to millennial scale modulation of ENSO amplitude is
consistent with evidence obtained from individual foraminifera in the EEP¹⁶ (Fig. 3f).

ENSO amplitude is known to be subject to large decadal to centennial internal unforced
variability ⁸,³⁷,²⁵,⁴⁷ but is, during the deglaciation, consistently higher during more than
4000 years, compared to its modern level. The length of these interannual records (both
of the Late Holocene and of the last deglaciation) yield an exceptional statistical
robustness to this reconstruction of ENSO (for comparison, Holocene fossil corals and
mollusks of the whole Pacific altogether record ~2500 years²⁵). A robust increase of
ENSO amplitude over such a long period could not be caused by stochastic internal
variability. Therefore, the larger deglacial amplitude of ENSO must have been the result
of a dynamical response to different boundary conditions.
<figure><img src="image_3.png" /><figcaption>Figure 2. A) Ti-XRF counts normalized record in Callao (red) and Pisco (blue). B) 2.5-8 years frequency band of the Ti-XRF record in Callao (red) and Pisco (blue). C) Standard deviation (over 300 years moving windows) of the 2.5-8 years frequency band of the Ti-XRF record in Callao (red) and Pisco (blue) as a proxy of ENSO variability</figcaption></figure>

ENSO and forcing mechanism during the last deglaciation

During the deglacial time, ENSO could have been affected by a number of forcing factors,
including insolation, greenhouse gases, the presence of large polar ice sheets and
meltwater discharge. The change in the seasonal flux of solar energy related to the Earth's
orbital parameters has been shown to be a driver of ENSO in models 48,49,50 and to account
for Holocene ENSO minimum (3-6 kyr BP) in paleoclimate records25. The damping effect
of this external forcing was, however, stronger during the deglaciation and could therefore
not explain the high variability recorded here (Fig 3a).

How increasing greenhouse gases influence ENSO is still unclear. While CMIP5 and
CMIP6 models together tend to suggest an increased ENSO variability with increasing
atmospheric CO2 concentrations⁵¹,⁵²,⁵³, two recent studies found that ENSO is weakening
with increasing CO2 when simulations last several millennia⁸ or when model biases are
minimized with an ultra-high-resolution model⁹. The 80 ppm increase of CO₂ during the deglaciation is slightly lower than the current 100 ppm anthropogenic CO₂ increase that has not produced a clearly detectable impact on ENSO, and is much lower than the x2 or x4 scenarios tested for the future that yields contradictory effects. Thus, CO₂ changes could arguably be considered as a minor forcing of ENSO during the deglaciation.

In contrast to the Late Holocene, the deglaciation is mainly characterized by large remnant continental ice sheets and massive meltwater discharge¹⁵,⁵⁴,⁵⁵,⁵⁶,⁵⁷. The presence of a large ice sheet in the northern hemisphere strengthens the mid-latitude jet stream and remotely affects the tropical Pacific through atmospheric bridges and had a slight strengthening effect on ENSO in the EP in the IPSL-CM4 climate model²³. Conversely, in the NCAR-CCSM3 transient simulation of the past 21,000 years forced uniquely by continental ice sheet variations, an abrupt ENSO increase (25%) is found at 14 kyr BP in response to a marked retreat of the Laurentide ice sheet⁵⁸. This effect disappeared when all forcing were combined⁵⁸. On the other hand, a meltwater discharge into the northern Atlantic consistently causes an increase of ENSO amplitude in multiple modeling experiments²⁰,²¹,²² and an eastward shift of anomalies²²,²³,⁵⁹, which would also result in increased variability in Peru. The 120 m sea level rise during the deglaciation⁶⁰ gives a measure of the magnitude of the corresponding meltwater flux into the North Atlantic and the Southern Ocean. Despite variations and some abrupt discharge events⁶¹, the sea level never stopped rising from 20 to 7 kyr BP. The sustained meltwater flux during the whole deglaciation period seems thus the most likely forcing of the multi-millennial increase in ENSO strength recorded in Peru.

The AMOC is weakened by meltwater discharge in the North Atlantic and in the Southern Ocean⁶²,⁶³,⁶⁴. The resulting change in the meridional-oceanic temperature gradient induces a southward displacement of the mean annual position of ITCZ, a weaker zonal
SST gradient across the equatorial Pacific and more symmetric annual mean climate in the EEP resulting in an amplified ENSO<sup>20,21,22</sup>. Although the underlying mechanisms responsible for the change in ENSO amplitude is still not clearly understood, the chain of response from the North Atlantic to the tropical Pacific is robust in model experiments.

Geochemical proxies of ocean circulation in marine sediments indicate a slowdown of the AMOC during the deglaciation due to large meltwater flux<sup>65,66,67</sup> (Fig. 3b). Records of tropical rainfall also point to a southern position of the mean annual position of the ITCZ in the deglaciation compared to the Late Holocene<sup>43,44,45</sup>. In addition, reduced zonal and meridional SST gradients were reconstructed in the Tropical Pacific (Fig. 3c) by Sadekov *et al.*<sup>16</sup> who also showed a strong link with ENSO amplitude. The deglacial climate, although rapidly changing, maintained an El Niño-like mean state in the tropical Pacific under the sustained influence of the meltwater flux weakening the AMOC. Our 4000-year long record shows that this mean state was associated with a long-term increase in El Niño amplitude in the EP, and provides observational support for the positive influence of meltwater flux on ENSO amplitude suggested by climate model experiments.

Very few model experiments have, however, explored the combined influence of multiple external forcing on ENSO during the deglaciation. The behavior of ENSO has been examined in the CCSM3 model TraCE-21ka transient simulation of the past 21 thousand years forced by orbital parameters, greenhouse gases, continental ice sheets and meltwater discharge, combined and separately<sup>58</sup>. Although meltwater discharge alone produces an increase in ENSO amplitude in the model, a compensation by the dampening effects of atmospheric CO<sub>2</sub> and insolation in TraCE-21ka results in a reduced ENSO during the last deglaciation<sup>58</sup> (Fig. 3e). Although TraCE-21ka reproduces correctly large-scale features of the evolution of the global climate (e.g., AMOC intensity, cross-Equator SST contrast, tropical Pacific SST)<sup>58</sup>, the simulated deglacial reduction of ENSO is at odd
with the increased amplitude recorded in Peruvian sediments. This disagreement indicates an overestimation of the dampening effect of CO₂ and insolation and/or an underestimation of the response to meltwater discharge in this model, an issue that is possibly shared by other climate models.

An underestimation of the impact of meltwater discharge on the tropical climate variability in climate models raises serious issues in the context of current global warming. There is multiple evidence for a slowdown of AMOC in the 20th century, possibly as a result of Greenland ice sheet melting<sup>68,69,70,71</sup> which is accelerating with global warming<sup>72</sup>. The future weakening of AMOC was considered as very likely in IPCC AR5<sup>73,74,75</sup>. The impact of AMOC slowdown on ENSO has been so far overlooked in climate change projections, possibly because of uncertainties in icesheet melting rate, or undetected for an underestimated response in projections. However, our record of ENSO in the EP during the last great ice sheet melting period suggests that ENSO is more sensitive to freshwater flux than previously thought, pointing to a potential strengthening with future ice sheet melting in a greenhouse climate. Additional transient simulations of the last deglaciation are urgently needed to better assess the ability of models to predict ENSO response.
<figure><img src="image_4.png" /><figcaption>Figure 3. A) December, January and February minus June, July and August (DJF–JJA) insolation at 30°S<sup>76</sup>. B) Composite <sup>231</sup>Pa/<sup>230</sup>Th record that reflect past changes in AMOC<sup>6</sup>. C) Changes in zonal gradient across the equatorial Pacific<sup>16</sup>. D) Meltwater flux forcing (m/kyr) in TraCE-21ka<sup>58</sup>. E) ENSO variability in only meltwater flux experiment (black) and ENSO variability in all forcing experiment in TraCE-21ka (green)<sup>58</sup>. F) Individual foraminifera Mg/Ca variability<sup>16</sup>. G) ENSO-related interannual variability of river discharge in Callao (red) and Pisco (blue).</figcaption></figure>
# Methods

## Marine sediments core

The cores M77/2-005-3 and G14 were retrieved from the Southeast Pacific continental slope during the M77-2 and Galathea-3 expedition respectively<sup>77,78</sup> (Salvatteci et al., 2016, 2019). The lithology of both cores, previously described by Salvatteci et al.<sup>77,78</sup> is here complemented by X-Ray images and presented in supplementary Fig. 2 and 3. The Late Holocene and deglacial sediments are mainly composed of “irregularly spaced laminae” and “isolated laminae”, as defined by Brodie and Kemp<sup>79</sup>. The “irregularly spaced laminae” are packets of laminae (several centimeters to decimeters thick) separated by intervals of homogeneous sediments (several centimeters to decimeters thick). These laminae packages include alternations between diatom oozes and diatomaceous mud, the latter with a higher content of clay minerals. The “isolated laminae” are packets of millimetric and sub-millimetric laminae (solitary diatom ooze) enclosed in homogeneous mud. The G14 core is more finely laminated than the M77/2-005-3 core. Depth-age models based on 26 <sup>14</sup>C ages for the core M77/2-005-3 and 29 <sup>14</sup>C datings for G14 was built by Yseki et al.<sup>19</sup> and is presented for both cores in supplementary Fig. 1. The analyzed section (36-588 cm) of core M77/2-005-3 included the last deglaciation (17-13 kyr BP; 102-588 cm), and, above a sedimentation hiatus at 94 cm, part of the Late Holocene (2.7-1.4 kyr BP; 36-93 cm). Probably because of erosion, Holocene sediments were not recovered in G14. The analyzed section (10-285 cm) of core G14 included the last deglaciation (17.5-13.5 kyr BP).

## XRF analysis

Ti was measured at high resolution (1 mm) in both cores using a XRF scanner only in the laminated or banded sections of the sediment cores in order to guarantee that the analyzed sections are a result of deposition from the water column and not from reworking of
upslope deposits<sup>77</sup>. XRF analysis were performed on core M77/2-005-3 and G14 at ALYSES facility (IRD-Sorbonne University, Bondy, France) and the University of Bordeaux 1 respectively. Measurements were done every 1 mm at 25kV and 500 uA during 10 s in core M772-005-3, and every 1 mm at 10kV and 400uA during 10 s in core G14.

Ti is present in marine sediments as titanium oxide resulting from soil pedogenesis and transported by rivers to the ocean along with clay minerals<sup>80</sup>. Ti-XRF has therefore been used as an indicator of river discharge, precipitation and ENSO variability<sup>81,82</sup>. XRF counts of Ti as well as other elements do not only depend on the mass concentration of Ti. It is also influenced by sediment heterogeneity that creates irregularities of the scanned core surface and mostly sediment inhomogeneity along the core such as variations in grain size or water content<sup>83</sup>. It is also influenced by the presence of elements that cannot be measured by this method, such as organic matter, causing a dilution effect, and more generally by matrix effects which includes the interactions of other elements present in the sediment on the XRF measurement of a given element<sup>83</sup>. For these reasons, the ratio or log ratio of XRF counts is frequently used in XRF core studies to correct for trends in sediment properties<sup>38,84,85</sup>. However, in the context of this study, which focuses on the high frequency variability of a sedimentary signal related to river inputs, considering the XRF counts of Ti in relation to another element would introduce an unnecessary noise that would disturb our analysis of the proxy fluctuation.

Centennial to millennial scale variations of fluvial and aeolian input during the last deglaciation were inferred from the grain size distribution in sediments from cores M77/2-005 and G14<sup>19</sup>. Downcore Ti-XRF variations in cores M77/2-005-3 and G14 covary with changes in fine particles of fluvial origin<sup>19</sup>. Maximum (minimum) Ti-XRF values occur when fluvial inputs increase (decrease) (supplementary Fig. 6), suggesting
that Ti-XRF is associated with particles of fluvial origin and not of aeolian and can be
considered a potential proxy for fluvial discharges. To allow for the comparison of XRF
data from different laboratories and different cores, a normalization to the maximum
value of titanium counts was performed (supplementary Fig. 4).

Extracting ENSO amplitude

The XRF analyses with 1 mm steps yield a temporal resolution of 0.8 years in the deglacial sections of core M77/2-005-3, and 2.2 years in the Late Holocene section (2.7-1.3 kyr). The temporal resolution of the XRF record in the deglacial section of core G14 is 1.3 years.

The records were resampled by linear interpolation at 0.5-year resolution in the deglacial and Late Holocene sections (supplementary Fig. 5). A 2.5-8 years band pass filter was then applied to all records to extract the ENSO frequency band. The ENSO band was calculated based on the difference of moving averages. The amplitude of ENSO-related variability was estimated by the standard deviation of the filtered signal over 300-years moving windows. The relative change of ENSO variability in Callao and Pisco was calculated in reference to the Late Holocene record from Callao. The reproducibility of the normalized Ti-XRF records and of the filtered Ti-XRF standard deviation in Callao and Pisco allows us to use the Callao Late Holocene section as a common modern reference.

Acknowledgements

This publication was supported by the IRD-DPF and ANR-15-JCLI-0003-03 BELMONT
FORUM PACMEDY. Part of XRF data were obtained on the ALYSES facility (IRD-
UPMC) that was supported by grants from Région Ile-de-France. This work is a
contribution of the Collaborative Research Project 754 "Climate-Biogeochemistry interactions in the Tropical Ocean" (www.sfb754.de), which is supported by the Deutsche Forschungsgemeinschaft (DFG). We would like to thank the crew and scientists aboard R/V Meteor cruises M77/2 in 2008 and thank Bo Thamdrup, chief scientist of the Galathea-3 expedition (Leg 14), and Bente Lomstein, who conducted the core sampling onboard the RV Vaedderen.

Author contributions

MY, BT and MC designed the study. MY wrote the manuscript with greatest input of
MC. RS, PM and HB measured XRF data. MY and DEM analyzed data. All authors
contributed to the interpretation of the data and the preparation of the final manuscript.

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Supplementary Files

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* Ysekietal2022ENSO.xlsx

• Ysekietal2022ENSOSI.pdf