Kunkelova et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-018-0238-x(2018) 5:86

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

A two million year record of low-latitude
aridity linked to continental weatheringfrom the MaldivesTereza Kunkelova1, Simon J. A. Jung1, Erica S. de Leau1, Nick Odling1, Alex L. Thomas1, Christian Betzler2,
Gregor P. Eberli3, Carlos A. Alvarez-Zarikian4, Montserrat Alonso-García5, Or M. Bialik6, Clara L. Blättler7,
Junhua Adam Guo8, Sébastien Haffen9, Senay Horozal10, Anna Ling Hui Mee3, Mayuri Inoue11, Luigi Jovane12,
Luca Lanci13, Juan Carlos Laya14, Thomas Lüdmann2, Nagender N. Bejugam15, Masatoshi Nakakuni16, Kaoru Niino17,
Loren M. Petruny18, Santi D. Pratiwi19, John J. G. Reijmer20, Jesús Reolid2, Angela L. Slagle21, Craig R. Sloss22,
Xiang Su25, Peter K. Swart3, James D. Wright24, Zhengquan Yao23, Jeremy R. Young26, Sebastian Lindhorst2,
Stephanie Stainbank27, Andres Rueggeberg27, Silvia Spezzaferri27, Igor Carrasqueira12, Siyao Yu24 and Dick Kroon1*

Abstract
Indian-Asian monsoon has oscillated between warm/wet interglacial periods and cool/dry glacial periods withperiodicities closely linked to variations in Earth’s orbital parameters. However, processes that control wet versus
dry, i.e. aridity cyclical periods on the orbital time-scale in the low latitudes of the Indian-Asian continent remain
poorly understood because records over millions of years are scarce. The sedimentary record from International
Ocean Discovery Program (IODP) Expedition 359 provides a well-preserved, high-resolution, continuous archive oflithogenic input from the Maldives reflecting on low-latitude aridity cycles. Variability within the lithogenic componentof sedimentary deposits of the Maldives results from changes in monsoon-controlled sedimentary sources. Here, we
present X-ray fluorescence (XRF) core-scanning results from IODP Site U1467 for the past two million years, allowing
full investigation of orbital periodicities. We specifically use the Fe/K as a terrestrial climate proxy reflecting on wet
versus dry conditions in the source areas of the Indian-Asian landmass, or from further afield. The Fe/K record showsorbitally forced cycles reflecting on changes in the relative importance of aeolian (stronger winter monsoon) during
glacial periods versus fluvial supply (stronger summer monsoon) during interglacial periods. For our chronology, we
tuned the Fe/K cycles to precessional insolation changes, linking Fe/K maxima/minima to insolation minima/maxima
with zero phase lag. Wavelet and spectral analyses of the Fe/K record show increased dominance of the 100 kyr cycles
after the Mid Pleistocene Transition (MPT) at 1.25 Ma in tandem with the global ice volume benthic δ18O data (LR04record). In contrast to the LR04 record, the Fe/K profile resolves 100-kyr-like cycles around the 130 kyr frequency bandin the interval from 1.25 to 2 million years. These 100-kyr-like cycles likely form by bundling of two or three obliquity
cycles, indicating that low-latitude Indian-Asian climate variability reflects on increased tilt sensitivity to regional
eccentricity insolation changes (pacing tilt cycles) prior to the MPT. The implication of appearance of the 100 kyr
cycles in the LR04 and the Fe/K records since the MPT suggests strengthening of a climate link between the low
and high latitudes during this period of climate transition.
Keywords: Maldives, IODP Exp 359, Non-destructive core scanning, Composition of lithogenic fraction, Orbital
cycles, Indian-Asian history of aridity, Mid Pleistocene transition* Correspondence: dick.kroon@ed.ac.uk
1School of GeoSciences, Grant Institute, University of Edinburgh, The King’s
Buildings, West Mains Road, Edinburgh EH9 3JW, UK
Full list of author information is available at the end of the article© The Author(s). 2018, corrected publication 2019. Open Access This article is distributed under the terms of the Creative
Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons license, and indicate if changes were made.

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

Introduction
Over the last two million years (Myr), the Earth’s climate
has oscillated between cold and warm periods driven bysemi-periodic changes of the Earth’s orbit around theSun. These changes forced waxing and waning of ice
sheets accompanied by global changes in sea surface
temperatures (SSTs), sea level, and extent of deserts inlow latitudes (Clemens et al. 1996). The general trend of
Earth’s climate shows a shift towards colder conditions,marked by a stepped increase in the amplitude of glacial
ice volume across the Mid Pleistocene Transition (MPT;
for an overview, see Head and Gibbard (2015), although
they used the term Early-Middle Pleistocene Transition).
The MPT, which began at about 1.25 Ma and ended
about 0.7 Ma (Fig. 1), is characterised by a change inglacial-interglacial cycles from a dominantly 41 kyr to a
100 kyr periodicity (Berger and Jansen 1994; Mudelsee
and Stattegger 1997). However, the emergence of the
100 kyr cycles is still a matter of debate and it is unclear
why the Earth’s climate system started to respond
non-linearly to orbital forcing (Maslin and Brierley

Page 2 of 202015). Elderfield et al. (2012) reported a rapid increasein ice volume during glacial stage 22 at about 0.9 Ma,
and during following glacial stages. Whereas temperatures decreased, and ice volume increased during glacial
periods post-MPT, ice volume and temperature maxima
remained relatively constant during the interglacial
periods for mid and low latitudes (Herbert et al. 2010;Rodrigues et al. 2017).
African terrestrial climate (wet-dry or aridity cycles)
records from International Ocean Discovery Program(IODP) drill sites around Africa similarly show a prominent switch in dominance from the 41 kyr to the
100 kyr periodicities during the MPT (deMenocal et al.
2000; Grant et al. 2017). However, to date, there has
been a conspicuous lack of long-term aridity records
from the Maldives. This area is situated in a particularly
desirable location as it receives large amounts of windblown dust derived from the surrounding Indian-Asian
landmass. Further, small amounts of fluvial material arederived from nearby India carried by currents originating in the Arabian Sea or Bay of Bengal. Therefore, aFig. 1 Earth’s climate variability in the last two Myr. a 65° N 21st June insolation. b Global ice volume benthic LR04 stack (Lisiecki and Raymo 2005).
c Global ice volume stack by Elderfield et al. (2012). Note the sudden increase in ice volume at stage 22 in the Mid Pleistocene Transition (MPT)Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

long-term lithogenic input record from the Maldives,
used as a proxy record for aridity linked to continental
weathering at source areas, provides a unique opportunity to investigate the Indian-Asian monsoon response to
the major global climate transition (MPT) over the last
two million years. IODP Expedition 359 (Betzler et al.
2017a) drilled several sites in the carbonate platform of
the Maldives archipelago (Fig. 2). The sedimentary
record of Site U1467, drilled in the drift sediments
associated with the atolls, provides a well-preserved,
high-resolution, continuous archive of lithogenic input
into the southern end of the eastern Arabian Sea.
We use X-ray fluorescence (XRF) core-scanning
records of lithogenic related element fluorescence intensities from IODP Site U1467 to evaluate influence oforbital cycles on variability in Indian-Asian terrestrial
climate related to monsoon activity over the last two
million years. In this study, we use Fe/K as a proxy ofaridity in marine sediments from the Maldives, wherehigh ratios indicate humid conditions and low values dry
conditions. The changes from high to low ratios reflect
wet-dry (aridity) changes in source areas, predominantly
on the Indian-Asian landmass; however, African sources
cannot be completely excluded. Fe/K proxy has beensuccessfully applied in marine sediments offshore Africa(Mulitza et al. 2008; Govin et al. 2012; Simon et al. 2016;
Ziegler et al. 2013). The two million year Fe/K record
from Site U1467 allows us to evaluate influence of regional orbital insolation and high-latitude ice volume
changes on low-latitude ‘wet-dry’ cycles across the MPT.

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Specifically, we document influence of the 100-kyr-like
cycles on amplitude of change in wet-dry (aridity) cycles
prior to the MPT. Tropical sea surface temperaturerecords (Liu et al. 2008) and tropical Pacific grain size
(wind strength) records (Yu et al. 2018) show presenceof late Pliocene to early Pleistocene 100-kyr-like cycles
formed by bundling of two/three obliquity cycles prior
to the MPT. Such late Pliocene/early Pleistocene
100-kyr-like cycles have not been previously reported
using wet-dry cycles from bore holes in the Maldives.

Lithogenic input in sediments of the Maldives
The sedimentary deposits in the Maldives contain lithogenic components derived from a mixture of sedimentsources. While dust grains form the largest component of
the detrital sediments, fluvial supplied material form theminor component. Today, the Maldives area is strongly
affected by the seasonally reversing Indian-Asian monsoon
system (Fig. 2): south-western monsoon winds dominate
between April and November, whereas north-eastern monsoon winds dominate between December and March(Wyrtki 1973). The summer monsoon brings strong precipitation over the Indian-Asian landmass, and high dust
flux into the western Arabian Sea (McDonald 1938;
Prospero 1981; Middleton 1986; Nair et al. 1989). The
winter winds bring cold air laden with dust over the ArabianSea. Modern satellite data show the aerosol distribution in
the Arabian Sea and the Maldives area, confirming the seasonal evolution of the dust plumes in 2017 (Fig. 3). While
the summer dust plume originating from the north-western

Fig. 2 Drilling location of Site U1467 in the Maldives Archipelago. The modern monsoon wind pattern over the Arabian Sea; showing thenorthwesterlies, the summer monsoon south-western winds and the north-eastern winter monsoon winds. The size of the arrows indicates
relative wind strength. Adapted from Leuschner and Sirocko (2003)

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

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Fig. 3 A compilation of aerosol distribution in the northern Indian Ocean during selected months during 2017 (Aerosol Optical Thickness, NASA,n.d.). Note that the winter dust plume reaches the Maldives

Arabian Sea scarcely extends into the eastern Arabian Sea
and the Maldives, the winter dust plume originating fromthe Indian-Asian landmass certainly reaches the Maldives.
Outflow of riverine material is the main source oflithogenic flux in the eastern Arabian Sea (Milliman etal. 1984). Fluvial-derived material in the Arabian Sea isdeposited in areas close to river deltas, e.g. the mouth of
the Indus, Narmada and Tapi rivers draining into theGulf of Khambhat and 16 small rivers draining theWestern Ghats (Ivanochko et al. 2005). Due to its bathymetric isolation, minor quantities of riverine material get
transported by currents to the Maldives (Kolla et al.
(1981). While some lithogenic material from the Arabian
Sea leaks southwards to the Maldives, north-eastern
winter currents also carry small quantities of finegrained fluvial-derived material from the Bay of Bengal
south and westwards to the Maldives (Kolla et al. 1981).
In summary, lithogenic material deposited in theMaldives is a mixture of dominantly aeolian transported grains from far afield and small amounts ofriverine material from the Indian continent and Bay of
Bengal. The new Site U1467 drilled in the Maldives
Inner Sea during IODP Expedition 359 is ideal for
documenting relative importance of wind-borne dust
(winter monsoon) and small fraction of fluvial-derived
material (summer monsoon) from India through time.

Methods/Experimental
The IODP Expedition 359 cored four holes at Site
U1467 in the Maldives archipelago, southwest of India

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

(4° 51′ N, 73° 17′ E), that is an isolated, partly
drowned tropical carbonate platform (Betzler et al.
2016, 2017a). The inner-atoll Site U1467 is a sedimentary drift sequence at a water depth of 487.4 m(Fig. 2). The sequence is continuous and preserves
the variability of the lithogenic fraction, recording a
regional history of aridity linked to the hydrologicalcycle. Betzler et al. (2017a) noted cyclical colourchanges in the sediment cores and linked these to theprimary orbital cycles. The nannofossil and planktonic
foraminiferal records confirm that the Plio-Pleistocene
record is continuous and complete, with sedimentation rates of about 3 cm per thousand years (kyr)(Betzler et al., 2017b). A shipboard composite depth
record was established (Betzler et al. 2017b) andcorrelations between the various bore holes wereaccomplished using standard IODP procedures (Correlator
software version 2.01.1.). High-resolution compositing was
based on sediment lightness (L*) data in Holes
U1467A-U1467D. However, the shipboard splice wasadjusted after the cruise based on newly produced XRF
core-scanning data at the IODP core store repository in
Texas. The newly produced XRF core-scanning data areconsidered to be more accurate than the shipboard lightness (L*) data. Individual Fe content records in the various
boreholes are shown in Fig. 4, including the new composite
Fe content record against the new composite depth(mcd = meters composite depth). The new splice data,
including the splice intervals, core offsets, and tie points
between each borehole, are archived on the IODP LIMS
Database (http://iodp.tamu.edu/database/).

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Scanning electron microscope

The lithogenic component of the sediment was analysedwith a Carl Zeiss SIGMA HD VP field emission scanningelectron microscope (SEM) at the University of Edinburgh.
We analysed eight selected samples from glacial and interglacial intervals. Samples were treated with 3% ethanoic
acid to remove calcite and aragonite from the samples.
Remaining suspensions were centrifuged at 2500 rpm for
30 min and the residue pellet rinsed with deionised water.
This leaching procedure was repeated to maximise carbonate removal. The residue pellet was then re-suspended
in a few drops of ethanol and transferred onto an SEM
stub. Energy dispersive analysis, calibrated against a
Co metal standard, was used to establish the chemicalcomposition of the residue grains.
X-ray fluorescenceNon-destructive X-ray fluorescence (XRF) scans were
measured at the Ocean Drilling and Sustainable Earth
Science (ODASES) XRF scanning facility at the IODP
Gulf Coast Repository (GCR) in Texas A&M University using a third-generation AvaaTech XRF scannerconfigured to analyse split sediment core halves forelements between Mg and U in the periodic table(Lyle and Backman 2013). Data were acquired with a
Canberra X-PIPS silicon drift detector (SDD) with150 eV X-ray resolution at 5.9 keV and a Canberra
Digital Spectrum Analyser model DAS 1000. The
X-ray source was an Oxford Instruments 100 W
Neptune X-ray tube with a rhodium (Rh) target. Raw
spectral data were processed using the Canberra

Fig. 4 The composite of Fe intensity (cps = counts per second) record comprised of segments from Holes B and C; CCSF core composite depth
below sea floor

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

WINAXIL software package to produce elemental
intensity data. The dual slit system was set to provide
down-core spatial resolution of 10 mm and cross-core
spatial resolution of 12 mm. The system was set toperform two consecutive runs of the same section,
the first one at 9 kV, 0.25 mA and 6 s, and thesecond one at 30 kV, 1.25 mA and 6 s. The first scan
provides data for the elements Mg, Al, Ba, Ca, Cl, Cr,
Fe, K, Mn, P, S, SI, Rh and Ti, and the second for Bi,
Cu, Br, Fe, Ga Nb, Ni, Mo, Pb, Rb, Sr, Y, Zn and Zr.
Each core section was removed from refrigeration atleast 2 h before scanning and scraped to clean andsmooth the core surface. The split core surface wascovered with 4 μm thick Ultralene plastic film toprevent contamination of the X-ray detector.
Measurements were taken at 3 cm intervals wheneverpossible. Because measurements cannot be performed
if the sediment presents cracks or uneven surface,some measurements were skipped or shifted to thenearest suitable area.
Samples from a selected interval (Cores 359-U1467C4H to -6H) were analysed using both the AvaaTech XRF
core scanner at Texas A&M and a conventional XRF
instrument at the University of Edinburgh. For these 1 g
aliquots of core material were ground in an agate mortar
and pestle and then pressed into 1 cm diameter pellets
using a pellet press operating at a pressure of 2 t/cm2. Pellets were analysed on a Philips PW2404 XRF spectrometerwith a Rh-anode X-ray tube. Corrections for matrix effects
on the intensities for major element lines were made using
theoretical alpha coefficients calculated using Philips software (Reynolds 1963). Intensities of the longer-wavelength
trace element lines were corrected for matrix effects usingalpha coefficients based on major element concentrations.
For other trace elements, matrix corrections were applied
using the Rh K alpha Compton scatter line as an internal
standard. Line overlap corrections were applied using synthetic standards. The spectrometer was calibrated with 15
USGS and CRPG standards using the values given in
Govindaraju (1994). Data derived from this procedure arereported both as raw count rates (for direct comparison
with the AvaaTech XRF core scanner at Texas A&M) andas wt.% oxides for major elements and ppm for trace elements for more routine data presentation. The pellets, analysed for major element and trace element composition at
Edinburgh, were subsequently sent to Texas A&M University for analysis using the same AvaaTech XRF core scanner. This procedure facilitated direct comparison of thescanned results from the two instruments, evaluation of
uncertainties in the XRF scanning counts, and calibrationof the AvaaTech XRF output for individual elements whereappropriate. All data will be also archived in Pangaea.
To evaluate the reliability of Fe intensities scannedwith XRF core scanner, the same subset of core (Cores

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359-U1467C-4H to -6H) was selected and sampled at a
3 cm interval. This enabled a comparison between the‘wet’ dataset acquired by XRF core scanner and the fully
quantitative ‘dry’ dataset by conventional XRF. The
samples were finely ground and made into pellets foranalysis by wavelength dispersive XRF at the Universityof Edinburgh. Pellets were analysed for a range of major
and trace elements. Plot of Fe content from conventional XRF of dry ground pellets versus the equivalent
position of Fe intensities in the ‘wet’ core by XRF corescanner shows a reasonable correlation coefficient(R2 = 0.7). Regression of this relationship allows forconversion of the ‘wet’ core scanner Fe count rates into
Fe wt.%. We calculated flux of Fe (gm−2 year−1) bymultiplying linear sedimentation rate by dry bulk
density (Betzler et al. 2017b) and Fe intensities (cps).
Spectral and wavelet analyses

Multitaper method (MTM) spectral analysis, data filtering, coherency and wavelet analysis with a Morlet wave
transform were performed on the Fe intensity, Fe/K
records and for comparison the LR04 δ18O stack record(Lisiecki and Raymo 2005). The MTM spectral analysis
was performed using the Astrochron package by Meyers(2014) in R, data were smoothed using a three-point
moving average (excluding LR04), normalised anddetrended. Analyseries 2.0.8. (Paillard et al. 1996) wasused to perform filtering and coherency analysis;
frequency (f ) and bandwidth (ban) parameters used fordata filtering were eccentricity (f = 0.0105, ban = 0.001),
obliquity (f = 0.024, ban = 0.03) and precession (f = 0.048,
ban = 0.006). Wavelet analysis was produced using Past
software (Hammer et al. 2001).

Results
Scanning electron microscope

Primary observations by SEM show the presence of dust
particles. The dust particles are easily recognised
because of their abraded surfaces resulting fromparticle-particle collisions during aeolian transport. SEM
images of both glacial and interglacial samples revealed
that dust particles (Fig. 5) are more abundant in thefraction of the insoluble residue of the glacial samples.
Point chemical analyses, and morphology revealed thepresence of predominantly quartz grains and, more
rarely, dolomite in the samples. The quartz grains show
the abraded, rounded surfaces, whilst the dolomite
grains show euhedral habit in situ growth.
X-ray fluorescence

Confidence in the AvaaTech XRF scanner results forindividual elements is assured by directly calibrating
the scan data with data from a conventional XRF
instrument permitting a robust, quantitative

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

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Fig. 5 Dust grains in various samples uncovered by scanning electron microscopy. SEM images showing selected dolomite (a) and quartz (b–d)
grains. The dust particles are recognised because of their abraded surfaces; scale bar = 100 μm

assessment of the scanning results for individual elements. We recognise that the lithogenic content of thesediments at Site U1467 is low (several %) which may
affect the reliability of the AvaaTech XRF scanning
output. The carbonate platform and pelagic carbonate
production is high in tropical settings diluting thelithogenic component and producing low concentrations of the terrigenous elements (Fe, K, Al, Si, Mg
and Ti). As the conventional XRF technique used with
the Philips PW2404 spectrometer is formallycalibrated using internationally recognised standards,
and the samples homogenised, the resulting data are
more robust. Therefore, we measured elements insamples from a selected interval to calculate correlation coefficients between the two datasets based onscanning counts.
We measured 67 samples from Cores 359-U1467C4H to -6H on both XRF instruments. These samples
cover the full range of lithologies observed at Site
U1467, from almost pure carbonate to clay-richcarbonates. The pellets were first measured on theconventional XRF instrument in Edinburgh and thesame pellets were sent to College Station in Texas andmeasured on the AvaaTech XRF core scanner. This
procedure facilitates calculation of regression lines andcorrelation coefficients for the individual elements.
Correlation coefficients range from about 0.2 to 0.9.
Ti shows the lowest value of about 0.2, whereas Fe
has a value of about 0.80, and Al has a value of 0.85.
K and Si have the highest values of about 0.9. These
results suggest that on the one hand AvaaTech XRF
core scanner results for Ti are not robust, but on theother hand they seem reasonably reliable for Fe andAl, and they seem very robust for K and Si. The reason for unreliability of Ti measured by the AvaaTechcore scanner is the low concentrations of Ti in thesediments. Although core scanner results seem reasonably robust for Fe, Al, K and Si, it is not clear at
this point whether the core scanner reliably measuresamplitude of change for individual elements. Water

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

content, grain size, porosity and other unknown factors may influence the element intensities (Kido et al.
2006; Koshikawa et al. 2003; Tjallingii et al. 2007). Alldata measured by the XRF instruments at Texas
A&M University and the University of Edinburgh arelisted in Additional file 1: Table S1.
Element intensities are plotted against composite
depth in Fig. 6. The Fe, K, Si and Ti records all show
cyclical changes throughout the record. The cyclicalchanges are related to primary Milankovitch frequencies given the general sedimentation rate of 3 cm/kyrof the investigated interval based on biostratigraphy(Betzler et al. 2017b). As such, the cyclical changes relate to the glacial-interglacial cycles dominating theclimate system in the last 2 Myr, with high and low values
representing sediment deposition during glacial andPage 8 of 20

interglacial periods, respectively. Marine isotopic stages(MIS) can be readily assigned to the individual cycles inthe depth domain (Fig. 6). All element intensity records show a conspicuous long-term upwards trend
from, on average, low to high values, and from small
to large changes in amplitude. Conspicuous increases
in the element intensity records occur during MIS
20, 22 and 24 within the MPT interval. This is particularly evident in the Fe intensity curve. The highest values of all element intensities can be found in
the younger glacial periods of MIS 2, 6 and 8. Theinterglacial values of element intensities records are
low throughout the sequence. The lowest values canbe found in the early part of the record, particularly
MIS 31, 49, 55, 57, 61, 63 and 73, although thesevalues are not much lower than some of the minimal

Fig. 6 The element intensity (cps = counts per second) time-series of terrigenous input transported by wind versus shipboard composite depth
as measured by the non-destructive XRF core-scanner at College Station, Texas A&M University. MIS are shown, dots indicate measured XRF corescanner data points; a three-point moving average smoothing was applied (bold line) to highlight the cycles

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

values in specific interglacial intervals such as in
MIS 7 and 9 in the younger part of the record.
To evaluate the AvaaTech XRF core scanner results interms of lithogenic variations, we aim to present element
ratios to remove dilution effects of carbonate (Shimmield
and Mowbray 1991; Schneider et al. 1997). Commonly,
element/Al ratios are used to remove the effect of variability in carbonate content reflecting biogenic production ofmarine sediments. Although this seems a straightforward
procedure, previous studies (e.g. Kido et al. 2006; Koshikawa et al. 2003; Tjallingii et al. 2007) have shown thatwater content, grain size, porosity and other unknown factors may influence the element intensities. Light elements,
such as Al, are more susceptible to these effects
compared to the heavier elements, which are affected
to a lesser extent and thus more reliable. To evaluate whether element/Al ratios as measured by thecore scanner are affected by these factors, we compared the element/Al core scanner results with theconventional XRF results (not shown here) in thedepth domain. The two data sets show large disparities, indicating that Al is heavily affected by watercontent and other factors of the sediment. Therefore,
element/Al is not suitable for the purpose of thisstudy.
We tested the validity of the Fe/K aridity proxy byplotting the conventional XRF and ‘wet’ and ‘dry’ scanner
results in the depth domain of a piece of the record(Fig. 7), whereby the same pellets were used for theconventional XRF and core scanner. To mitigate the effects
of slight sediment displacement within the core as a resultof sampling of discrete samples and scanning, that weredone on the working half and archive half, respectively, weused a three-point running average through the data.
The ‘wet’ and ‘dry’ Fe/K profiles are very similar,
implying that water content and grain size changes
do not substantially affect the record. The conventional XRF results also show a similar profile. The
orbital cycles are noticeable, although some disparities
occur. Overall, we conclude that the measurements of
Fe/K are not perfect but reasonably reliable. Therefore, we apply the Fe/K ratio to the entire sedimentrecord representing two million years.

Chronology
The AvaaTech XRF-based scanning records of element
intensity and Fe/K show the presence of orbital cyclicity
in element concentrations and lithogenic particle
composition. We use the Fe intensity and Fe/K records
for tuning to insolation cycles, in order to generate an
age model for the sequence. All marine isotope stages
from MIS 1–73 can be recognised (Fig. 8), but instead ofcorrelating the Fe intensity and Fe/K cycles with thebenthic stable isotope stack (LR04), which is based on a

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compilation of benthic δ18O records from multiple sites(Lisiecki and Raymo 2005), the Fe intensity and Fe/K
records were directly tuned to the precession parameterof orbital variability (Laskar et al. 2004). Significantly,
when the Ca/K (carbonate productivity versus lithogenic
input) is compared with the corresponding Fe/K curve,
the same temporal relationship with insolation emerges(Fig. 8). Direct tuning to precession means that no inference can be made regarding climate and precession, but
it allows for the investigation of longer periodicities of
eccentricity and obliquity. This tuning method is based on
the assumption that aridity in the source areas respond
more readily to insolation, although may have an influence
from northern hemisphere climate (Clemens et al. 1991).
The value of this approach is that it is independent of theLR04 record. It also removes any age offset assumption of
the LR04 ice model to insolation forcing. The Fe/K record
displays orbital-driven cycles in tandem with the Fe intensity record. The highest amplitude of change of the cycles
is in the early part of the record prior to the MPT.
The Fe intensity and Fe/K records were smoothedusing a three-point moving average. Cycles related toprecession are visually pervasive throughout the record.
These cycles were directly tuned to the precessional
component of the 65° N 21st June insolation curve using
the La2004 orbital solution (Fig. 8; Laskar et al. 2004).
We tied the Fe intensity minima and Fe/K maxima toprecession minima (insolation maxima) with zero phaselag using Analyseries 2.0.8 software (Paillard et al.
1996). All tie-lines are given in Additional file 2: TableS2 and the age model, composite depth (mcd) and element intensity values are given in Additional file 3: Table S3.
Some studies have used a more local low-latitude insolation curve; however, as both are calculated on 21st of June,the insolation peaks and troughs occur simultaneously.
Thus, the age model is not affected by the choice of thedifferent insolation latitude. To illustrate this point, weplot both the 21st June insolation records at 65° and 10° N
alongside the Fe intensity and Fe/K records in Fig. 8.
To assess the validity of the precession-based chronology, we have filtered the orbital components of the Fe/
K time series (Fig. 9). In addition to the precessional
component, which should include the artefact of thetuning approach, the filtered components of obliquity
and eccentricity from the Fe/K record are shown incomparison to the respective obliquity and eccentricity
components of the 65° N 21st June insolation curveusing the La2004 orbital solution (Laskar et al. 2004).
This confirmation of the presence of eccentricity andobliquity periods in the filtered respective records confirms our approach of using precession cycles to obtaina chronology of the record. It is particularly the pervasive presence of the obliquity cycles that lends increased
confidence in our age model.

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Fig. 7 Comparison of original ‘wet’ and ‘dry’ Fe/K data, measured directly on the core at College Station with ‘dry’ Fe/K data obtained from dried andground core samples measured at the University of Edinburgh. Points indicate original data; black line represents a three-point moving averageKunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

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Fig. 8 The Fe intensity, Fe/K and Ca/K records tuned to the 65° N 21st June insolation curve by Laskar et al. (2004). The 10° N 21st of June andMIS are shown; tie points between the records are indicated in Additional file 1: Table S1, a three-point moving average smoothing was applied
to highlight the cycles. Note that the y axis on the Fe intensity record is reversed. Sedimentation rates vary between 1.15 and 7.7 cm/kyr

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

Page 12 of 20

Fig. 9 Filtered components of eccentricity (blue), obliquity (green) and precession (red) from the Fe/K record compared with the respective
orbital components by Laskar et al. (2004). Fe/K record is plotted in dark grey

Spectral and wavelet analyses of the Fe intensity, Fe/K
and LR04 records

Spectral (Fig. 10) and wavelet (Fig. 11) analyses of
the Fe intensity, Fe/K and LR04 records show pervasive presence of the primary orbital cycles. The Fe/K
spectral and wavelet results display distinct power inthe precession frequency band, although this may
have been somewhat artificially induced because of
the age modelling approach. This is particularly
evident in the one to two million year time-window(Fig. 11). The Fe/K spectral and wavelet results also
show occurrence of tilt throughout the record,affirming the age modelling approach. Power of tilt
is less strong in the Fe/K record than in the LR04
record. An important feature of the wavelet diagramsis the presence of strong eccentricity (100 kyr) in
the 0–1.25 million year interval and occurrence of
faint eccentricity like (spectral power broadly ranging
around 130 kyr) in the 1.25–2 million year interval,
resulting in a distinct switch in the lower frequency
band that is eccentricity related around 1.25 million
years ago: spectral power of eccentricity-like cycles(~ 130 kyr) diminishes and proper eccentricity cycles(100 kyr) emerge.

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

Page 13 of 20

Fig. 10 Spectral analysis results using the Fe intensity (light red), Fe/K (dark red) and LR04 benthic stable oxygen isotope stack (blue) records. Also,
coherences between Fe intensity and LR04 (grey) and Fe/K and LR04 (black) are shown. Dashed lines indicate 95% confidence levels (see legend)

Discussion
IODP Expedition 359 drilled a continuous sedimentaryarchive in the Maldives, available for monsoonal
research (Betzler et al. 2017a, b). We set out to use
variability in the lithogenic component of the sediments
to study wet-dry cycles in the Indian-Asian source areasin the Plio-Pleistocene. We use the Fe/K as an aridity
proxy indicating cyclical changes between humid anddry conditions on the Indian-Asian landmass.
Moisture changes in source areas of the Indian-Asian
landmass

Variations in the composition of lithogenic material in
marine sediments are used to infer source area weathering, representing changes between humid and dry conditions driven by summer and winter monsoon activity,
respectively. We follow the assumptions by previous
studies that strong precipitation over the continent
during summer monsoons increases flux of fluvial
material, whereas aeolian dust flux increases during winter monsoon (Govin et al. 2012; Simon et al. 2016;
Ziegler et al. 2013). On long-term time-scales, changes
in the hydrological cycle, and subsequently changes in
the annual monsoon activity, is driven by orbitally forced
insolation variability (Clemens et al. 1991). Therefore,
we use the assumption that Fe/K ratios are an indicatorof long-term variability of fluvial versus aeolian flux,
where Fe represents chemical weathering of soils and K
represents the mechanical weathering. Thus, we conclude that high Fe/K ratios are indicative of periods ofincreased chemical weathering (humid climate), while
low Fe/K ratios indicate increased mechanical weathering (arid climate). At present, we do not precisely know
which minerals host the Fe component. However, in thetropical humid region of southern India, relatively hightemperatures and high precipitation (summer monsoon)
favour chemical weathering of bedrocks resulting inheavily lateritized bedrocks. These weathered soils areenriched in Fe oxides and clays which are transported assuspended matter by rivers to the India Ocean. Thus, we
relate the high Fe/K ratios in the Maldives record to a
relative shift towards more chemical weathering and fluvial activity in India during warm and wet interglacial
periods. This is further supported by clay mineral distribution along the Indian continental margin. Kolla et al.
(1981) documented high percentages of smectite clays
around the margin of India, which were supplied byrivers that drain the soils developed on the Deccan trap
basalts. The content of smectite along the western
margin generally decreases from north to south, but thepercentage of smectite significantly increases again near
the southern tip of India, which is a result of westward

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

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Fig. 11 Wavelet transform results of the Fe intensity, Fe/K time series and LR04 benthic stable oxygen isotope stack. The 95% confidence level
curve is shown (black)currents carrying smectite from the Bay of Bengal(Kolla et al. 1981; Wyrtki 1973). The Maldives as asediment trap receive these smectite clays (and byinference other Fe bearing minerals) because of low
salinity surface currents flowing from the Bay of
Bengal in winter (Kolla et al. 1981). Future work isneeded to investigate and confirm all possible sourcesof Fe in the study site; however, that is currently
beyond the scope of this paper.
In low-latitude dry regions (deserts), low temperatures and low precipitation during winter monsoon
promote mechanical weathering of bedrocks, resultingin K-rich illite and feldspars (Kolla et al. 1981). Thus,
we relate the low Fe/K ratios in the Maldives record toa relative shift towards more mechanical weatheringand aeolian activity in further afield desert areas
during cold and dry glacial periods. The most likely
illite sources during these cold and windy periods areof aeolian origin coming from the surrounding arid
regions further afield, such as the Thar desert andArabian Peninsula. Also, the Indus River was much
reduced during the glacial periods, and suspended
material was unlikely to reach the Maldives.
We selected a small number of samples (7) toinvestigate mineral content of the lithogenic fraction
using the X-ray diffraction method across the sediment record. The analyses showed substantial
percentages of minerals of the smectite group
showing that Indian-derived clays were deposited in
the Maldives. Montmorillonite (Fe) of the smectitegroup minerals is most abundant (up to 15%) and
importantly shows an inversed occurrence with illite(K) in the samples, reflecting weight percentages ofIndian-derived (fluvial) versus desert-derived (wind)clay minerals. We concede that XRD analysis should
be carried out across a number of cycles to verify

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

shifting mineral assemblages, but this is currently
beyond the scope of this paper.
We conclude that Fe/K ratios reflect changes in monsoonal climates resulting in aeolian versus fluvial transportmodes from the source areas to the ocean. High Fe/K ratios
in an interval of sediments indicate that increased chemical
weathering occurred during the wet summer monsoon season, coeval with heating of the continent, and that rivers
transported abundant suspended Fe to the ocean. Aeoliantransport of this Fe-rich material is unlikely due to theelevated moisture promoting vegetation growth and suppressing wind erosion. The alluvial flux of Fe to theMaldives will be further enhanced during wet periods as
not only will the sources from chemical weathering be
increased but also the higher precipitation will further reduce the salinity of the surface ocean currents bringing thesediment to the Maldives. Increased buoyancy of the surface ocean will inhibit mixing and sediment loss into thedeep ocean along the transport pathways. Conversely, low
Fe/K ratios in an interval of sediment indicate that increased mechanical weathering occurred during the dry
winter monsoon concomitant with cooling of the continent
in the source areas, and that strong winter winds transported abundant quartz (see Fig. 5), illite and K-rich feldspars to the ocean. We expected subtle changes in the Fe/K
ratios of the lithogenic fraction in the Site U1467 sediments, because only a small fraction of fluvial-derived
material arrives at the Maldives. In contrast, majority of thelithogenic fraction arrives as dust. We recognise that processes controlling the Fe/K are multiple and complex, butultimately, the amplitude of change in the Fe/K record
reflects relatively wet (chemical weathering) versus dry(mechanical weathering) conditions linked to the hydrological cycle driven by monsoon activity in various sourceareas of the adjacent continent or further afield.
We plot the Fe/K record together with the deep-sea
stable oxygen isotope stack (LR04; Lisiecki and Raymo

Page 15 of 20

2005; Fig. 12). The Fe/K cycles correlate to the marine
isotopic stages, with high and low values during interglacial and glacial stages, respectively (Fig. 12), reflecting on
orbital-driven wet versus dry cycles. The high Fe/K
values during interglacial periods indicate more humid
conditions (intense summer monsoon) and the low
values point to drier conditions during the glacial
periods (intense winter monsoon). Some of the ‘wettest’interglacial periods (MIS 55, 61 and 73) and ‘driest’
glacial periods (MIS stages 52 to 74) occur in the earlypart of the record prior to the MPT. One of the last
distinct ‘wet’ periods MIS 31 occurs during the MPT. To
add confidence in the interpretation of Fe/K cycles
representing wet-dry cycles in the hinterland, we alsoplotted the Fe flux record (Fig. 13, Additional file 4:
Table S4), which is a firmly established aridity proxy record (Clemens and Prell 1990, 1991). The Fe flux cycles
confirm the interpretation of the Fe/K record withhumid conditions during interglacial periods and dry
conditions during glacial periods (Fig. 13).

Two million year record of insolation-driven
Indian-Asian hydroclimate
The Indian-Asian climate is directly controlled bymonsoon activities, which are sensitive to regional
insolation changes driven by orbital precession,
although effects associated with high-latitude climatechanges may also play a role. The Fe/K cycles at Site
U1467 track northern hemisphere summer insolation,reflecting on moisture supply from the Arabian Sea.
Sensitivity experiments support the assumption that
positive summer rainfall shows a strong relationshipwith orbital precession (Clemens and Prell 1991).
Visual inspection of the Fe/K record revealed theglacial-interglacial cycles and precession-driven cycles.
On the basis of the presence of distinct precession
cycles (particularly during interglacials) and initial age

Fig. 12 The Fe/K record and the LR04 benthic stable oxygen isotope stack against age (kyr). A three-point moving average smoothing wasapplied to the Fe/K record

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

Page 16 of 20

Fig. 13 Clay and silt fraction (< 63 μm in %), bulk density (Grape data), Fe flux (gm−2 year−1), Fe intensity (cps = counts per second) and LR04
benthic stable isotope stack against age (kyr)modelling, we tuned the Fe/K record to precession.
Our approach was to correlate maximum Fe/K ratios
to insolation peaks (precession minima) and minimum
Fe/K ratios to insolation lows (precession maxima).
The spectral and wavelet analyses diagrams of the Fe/K
display distinct power in the precession frequency band. It
is particularly evident in the 1.25 to 2 million year
time-window (Figs. 10 and 11), where spectral power resolves the 23 kyr and the 19 kyr cycles. However, it is important to note that the direct tuning to precession may
have induced an artificial precession signal. The presence oftilt (41 kyr cycle) in the spectral analysis throughout therecord, on the other hand, confirms our age model approach. Other striking features of the spectral diagrams isthe presence of strong eccentricity (100 kyr) in the 0–1.25
million year interval and occurrence of faint eccentricity(spectral power broadly ranging from 80 to 140 kyr) in the

1.25–2 million year interval. Wavelet power analysis shows
similar orbital frequency features in the Fe/K record. The
low-frequency eccentricity cycles present prior to the MPT
have been previously described by Liu et al. (2008) in the
Ocean Drilling Project Site 722 sea surface temperaturerecord, defining them as eccentricity-like cycles likely
formed by bundling of two or three obliquity cycles resulting in saw tooth patterns. The same bundling of tilt cycles
is observed in the Fe/K record prior to the MPT. Liu et al.
(2008) postulated that such bundling of tilt cycles was
paced by eccentricity. We infer that combined presence oftilt (bundled in saw tooth shapes) and precession (amplified
by the 100-kyr-like cycle) is pivotal for understanding orbitally forced low-latitude humidity changes in the pre-MPT
period, resulting in extreme monsoon-driven climates. The
wettest interglacial periods (MIS stages 31, 55, 61 and 73)
and ‘driest’ glacial periods (MIS stages 52 to 74) occur inKunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

Page 17 of 20

Fig. 14 Temperature records based on alkenones from various boreholes in the Atlantic and Arabian Sea. Sites 607 and 982 were drilled in
the N-Atlantic; Site 1082 was drilled in the South Atlantic and Site 722 in the western Arabian Sea. A 200 kyr moving average smoothing wasapplied (bold line) to reveal the long-term trends. The early onset of a fall in temperatures in Sites 607 and 722 coincides with the early dropin Fe/K ratios in Site U1467 prior to the switch in dominance of the 41 kyr to the 100 kyr around 1.25 Ma. There is a strong resemblance of thelong-term trends in the Fe/K record with the temperature records

Kunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

this interval. MIS 31 is the maximum peak of the Fe/K
record, likely reflecting minimal dust and maximum
fluvial-derived inputs in the sediments of the Maldivescoinciding with the highest peak of the total insolation
profile in the last two million years.
The link between the Fe/K aridity record and the global
ice volume record

The Fe/K record shows a significant coherence with theLR04 benthic stable oxygen isotope record of mainly global
ice volume over all primary orbital periodicities (Lisiecki
and Raymo 2005; Fig. 12). This suggests that low-latitude
aridity and global ice volume changes are linked, reflecting
on common orbital forcing principles of climate. However,
the wavelet power analyses of the Fe/K and LR04 benthic
stable oxygen isotope records show a notable disparity(Fig. 11) concerning influence of eccentricity forcing. Whilethe eccentricity-like (130 kyr) cycles are distinct in the
pre-MPT period, revealed by considerable power in the
wavelet power analysis, there are no such eccentricity-like
cycles (or they are very faint) in the LR04 record. Notably,
power on the eccentricity (100 kyr) cycles in both the Fe/K
and LR04 records emerges around 1.25 million years, suggesting a strengthening climate link between the two areas.
The low-latitude monsoon system likely responded to thehigh-latitude ice sheet histories, or alternatively, both areas
responded to a common unknown climate driver emerging
at the time. Comparison of the long-term average change
in the Fe/K record to the Arabian Sea Site 722 and various
Atlantic sea surface temperature records reveals along-term shift towards drier conditions in the low latitudes
concomitant with sea surface temperature falls in the
Arabian Sea and high latitudes across MIS stage 22–24,
further suggesting a strengthening climate link between low
and high latitudes across the MPT (Fig. 14).
Lisiecki and Raymo (2005) used a simple ice model basedon 21 June insolation at 65° N with a set lag to insolation
allowing for ice sheet response time. This phase lag is adjusted during the late Pliocence to early Pleistocene from 5
to 15 kyr from 3.0 to 1.5, to represent the increasing ice
mass during this interval. In contrast, the aridity record of
the Maldives is directly tuned to insolation to represent thehighly sensitive response of Indian-Asian hydroclimate to
insolation changes. As a result of the different tuning approaches, our records of the Maldives lead the LR04
benthic stacked record (Fig. 12). The consistency of thisphase lead should serve as a test of the robustness of thetuning approaches of either the LR04 stack or our record.
The phase lag is evidently the case during deglaciations
throughout the record.

Conclusions
The Maldives sedimentary archive contains a valuable
lithogenic input record reflecting on terrestrial climate

Page 18 of 20changes in the source areas of the Indian-Asian landmass or from further afield. The Fe/K aridity record
shows orbitally forced cycles reflecting on changes in therelative importance of aeolian (stronger winter monsoon) during dry glacial periods versus fluvial supply(stronger summer monsoon) during humid interglacial
periods. The chronology is based on precessional cycles
visually present in the Fe/K record, where Fe/K maxima
correlate to precession minima (insolation maxima) withzero phase lag. The value of this approach results in anage model independent from the LR04 record.
Wavelet and spectral analyses of the Fe/K record show
increased dominance of the 100-kyr cycles throughout
the record across the Mid Pleistocene Transition (MPT)in concert with the benthic foraminiferal oxygen isotopic
δ18O data (LR04 record). The emergence of 100 kyr
cycles in both the LR04 and the Fe/K records since the
MPT suggests strengthening of a climate link between
the low and high latitudes during this period of major
climate transition. In contrast to the LR04 record, the
Fe/K profile displays 100-kyr-like cycles around the
130 kyr frequency band, prior to the MPT, in theinterval 1.25 to 2 million years ago. The influence(power) of the 100-kyr-like cycles weakens towards the
MPT. These 100-kyr-like cycles likely form by bundlingof three obliquity cycles. The development of the
100-kyr-like cycle between two and one million years indicates that low latitude Indian-Asian climate variability
reflects on increased tilt sensitivity to regional eccentricity insolation changes (pacing tilt cycles) prior to the
MPT, resulting in large amplitude changes of the Fe/K
record. As a result, both ‘wettest’ interglacial periods(MIS stages 31, 55, 61 and 73) and ‘driest’ glacial periods(MIS stages 52 to 74) occur in this interval.

Additional files
Additional file 1: Table S1. X-ray fluorescence comparison between
conventional XRF (University of Edinburgh) and XRF core-scanner (Texas
A&M University). (XLSX 12 kb)
Additional file 2: Table S2. Age model tie-lines. (XLSX 13 kb)
Additional file 3: Table S3. Composite splice (mcd), age (kyr), element
intensities (cps). (XLSX 169 kb)
Additional file 4: Table S4. Age (kyr), sedimentation rate, bulk density(Grape) and Fe flux. (XLSX 155 kb)

Acknowledgements
The authors would like to thank the crew of the JOIDES Resolution for theirassistance during the collection of the cores. D. Houpt is acknowledged forassistance during XRF scanning at IODP Gulf Coast Repository. R. Tada andtwo anonymous reviewers are thanked for their critical and constructive
comments which helped to sharpen our concept.
Funding
DK acknowledges support by the Natural Environment Research Council(NERC 3148), funding support for conventional XRF, core scanning andtechnical assistance. MAG acknowledges support provided by theKunkelova et al. Progress in Earth and Planetary Science(2018) 5:86

Portuguese National Science and Technology Foundation (SFRH/BPD/
96960/2013, PTDC/MAR-PRO/3396/2014 and UID/Multi/04326/2013),
funding support for core scanning.
Availability of data and materials
The datasets generated and/or analyzed during the current study areavailable in the Pangaea repository (https://doi.pangaea.de/10.1594/
PANGAEA.897951).
Authors’ contributions
TK led the writing of the manuscript, contributed to figures andstatistical analysis. SJAJ contributed to analysing the data and several
figures. ESL contributed to sample preparation for conventional XRF
scanning and statistical analysis. NO contributed to conventional XRF
scanning. ALT contributed to statistical analysis and design of several
figures. CB served as Co-Chief on the Maldives Leg, contributed to
shipboard handling and description of cores, contributed to corescanning. GPB served as Co-Chief on the Maldives Leg, contributed to
shipboard handling and description of cores, contributed to corescanning. CAAZ served as IODP manager on the Maldives Leg,
contributed to shipboard handling and description of cores, contributed
to core scanning. MAG contributed to shipboard handling of cores andmicropaleontology, contributed to core scanning. OMB contributed to
shipboard handling and description of cores. CLB contributed to
shipboard handling of cores and chemistry. JAG contributed to
shipboard handling and physical properties of cores. SHa contributed to
shipboard handling of cores and physical properties of cores. SHocontributed to handling of cores and physical properties. ALHM
contributed to shipboard handling and description of cores. MI
contributed to shipboard handling of cores and chemistry. LG
contributed to shipboard handling of cores and magnetostratigraphy,
contributed to core scanning. LL contributed to shipboard handling ofcores and magnetostratigraphy. JCL contributed to shipboard handlingand description of cores, contributed to core scanning. TL contributed
to shipboard handling of cores and geophysics. BNN contributed to
shipboard handling and description of cores. MN contributed to
shipboard handling and description of cores. KN contributed to
shipboard handling of cores and micropaleontology. LMP contributed to
shipboard handling of cores and micropaleontology. SDP contributed to
shipboard handling of cores and micropaleontology. JJGR contributed to
shipboard handling and description of cores, contributed to corescanning. JR contributed to shipboard handling and description of cores,
contributed to core scanning. ALS contributed to shipboard handling ofcores and core logging. CRS contributed to shipboard handling anddescription of cores, contributed to core scanning. XS contributed to
shipboard handling of cores and micropaleontology. PKS contributed to
shipboard handling of cores and chemistry. JDW contributed to shipboard
handling cores, correlated the boreholes, and contributed to core scanning. ZY
contributed to shipboard handling and description of cores. JRY contributed to
shipboard handling cores and micropaleontology. SL contributed to shore
based grain size analyses. SS contributed to core scanning. AR contributed tocore scanning. SS contributed to core scanning. IC contributed to corescanning. SY contributed to core scanning. DK contributed to shipboard handling of cores, micropaleontology and core scanning. All authors contributed towriting of the manuscript.
Competing interests
The authors declare that they have no competing interest.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in publishedmaps and institutional affiliations.
Author details
1
School of GeoSciences, Grant Institute, University of Edinburgh, The King’s
Buildings, West Mains Road, Edinburgh EH9 3JW, UK. 2Institute of Geology,
CEN, University of Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany.
3
Department of Marine Geosciences, Rosenstiel School of Marine &Atmospheric Science, University of Miami, Miami, FL 33149, USA.
4
International Ocean Discovery Program, Texas A&M University, Discovery
Drive, College Station, TX 77845, USA. 5Instituto Português do Mar e daPage 19 of 20

Atmosfera (IPMA), Divisão de Geologia e Georecursos Marinhos, Rua Alfredo
Magalhães Ramalhoi 6, 1495-006 Lisboa, and Centro de Ciências do Mar(CCMAR), Universidade do Algarve, Faro, Portugal. 6Dr. Moses Strauss
Department of Marine Geosciences, The Leon H. Charney School of Marine
Sciences, University of Haifa, 31905 Carmel, Israel. 7Department ofGeosciences, Princeton University, Guyot Hall, Princeton, NJ 08544, USA.
8
Department of Geological Sciences, California State University Bakersfield,
9001 Stockdale Highway, Bakersfield, CA 93311, USA. 9Ecole Nationale
Supérieure de Géologie, Université de Lorraine, 2 rue du Doyen Marcel
Roubault, 54501 Vandoeuvre-les-Nancy, France. 10Petroleum and Marine
Research Division, Korea Institute of Geoscience & Mineral Resoursces(KIGAM), Gwahang-no 124, Yuseong-gu, Daejeon 305-350, South Korea.
11
Graduate School of Natural Science and Technology, Okayama University,
3-1-1 Tsushima-naka, Okayama 700-8530, Japan. 12Instituto Oceanográfico da
Universidade de São Paulo, Praça do Oceanográfico, 191, São Paulo, SP
05508-120, Brazil. 13Department of Pure and Applied Science, Università di
Urbino, “Carlo Bò” Via S. Chiara 27, I-61029 Urbino, Italy. 14Department of
Geology and Geophysics, Texas A&M University, College Station, TX
77843-3115, USA. 15Geological Oceanography Division, CSIR-National Instituteof Oceanography, Dona Paula, Goa 403004, India. 16Department ofEnvironmental Engineering for Symbiosis, Soka University, 1-236 Tangi-cyo,Hachioji-shi, Tokyo 192-0003, Japan. 17Graduate School of Science andEngineering, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata
990-8560, Japan. 18Environmental Science and Policy Department, George
Mason University, David King Hall Rm 3005, MSN 5F2, University Drive,
Fairfax, VA 22030-4444, USA. 19Department of Geosciences, GeologicalEngineering Faculty – Universitas Padjadjaran, Jl. Raya Bandung Sumedang
KM 21, Jatinangor 45363, Indonesia. 20College of Petroleum Engineering andGeosciences, King Fahd University of Petroleum and Minerals, Dhahran 3126,
Saudi Arabia. 21Lamont-Doherty Earth Observatory, Columbia University,
Borehole Bldg. 61 Route 9W, Palisades, NY 10964, USA. 22Earth andEnvironmental Sciences, University of Technology Queensland, R-Block 317, 2
George Street, Brisbane, QLD 4001, Australia. 23Department of Marine
Geology, First Institute of Oceanography (FIO) State Oceanic Administration(SOA), #6 Xian Xia Ling Road, Qingdao, Shandong 266061, People’s Republicof China. 24Department of Geological Sciences, Rutgers, The State Universityof New Jersey, 610 Taylor Road, Piscataway, NJ 08854-8066, USA. 25Key
Laboratory of Marginal Sea Geology, South China Sea Institute of
Oceanology, Chinese Academy of Sciences, West Xingang Road, Guangzhou
510301, People’s Republic of China. 26Department of Earth Sciences,
University College London, Gower Street, London WC1E 6BT, UK.
27
Department of GeoSciences, Université de Fribourg, Ch. du Musée 6 1700,
Fribourg, Switzerland.
Received: 3 October 2017 Accepted: 8 November 2018

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