Usui and Yamazaki ﻿Progress in Earth and Planetary Science(2023) 10:47
https://doi.org/10.1186/s40645-023-00581-z

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open Access

K‑feldspar enrichment in the Pacific pelagic
sediments before Miocene
Yoichi Usui1,2*    and Toshitsugu Yamazaki3

Abstract
The mineralogy of atmospheric silicate dust controls its interaction with clouds. K-feldspar has a remarkably highice-nucleating activity, and its distribution may have influenced the global climate throughout Earth’s history. However, long-term archives of past atmospheric feldspar are not known. Here, we investigate feldspar mineralogy,
content, and grain size in pelagic clay cores. Sediments around Minamitorishima Island contain > 10 wt% of K-feldspar
before ~ 35 Ma, which is five times more than the younger sediments. This distribution does not resemble other volcanic minerals or geochemically estimated volcanic input, suggesting that the K-feldspars are not associated with volcanic ash. The K-feldspars are present as isolated grains as well as pseudorhombohedral microcrystals indicativeof authigenic overgrowth. On the other hand, they contain some Na, arguing against a purely authigenic origin. Grainsize distributions of chemically separated quartz and feldspars show stratigraphic variation analogous to other North
Pacific sites, further suggesting a link to eolian materials. Sediments from a South Pacific site also show K-feldspar
enrichment over plagioclase before ~ 44 Ma, although the content relative to bulk sediment does not change much.
We propose that the K-feldspar may be enriched in the wide area of the Pacific before ~ 30 to 40 Ma.
Keywords X-ray diffraction, Grain size, Minamitorishima, Site U1366, Ice nucleating particles
1 Introduction
There has been renewed interest in aerosol mineralogy
because of the remarkably high ice-nucleating ability of
K-feldspar (Atkinson et al. 2013; Harrison et al. 2016).
Even though K-feldspar often represents a small fraction (< 5%) of mineral aerosol today, the type, amount,and size of K-feldspar could control the heterogeneous
ice nucleation in mixed-phase clouds (e.g. Kanji et al.
2017). This process is critical for cloud microphysics*Correspondence:
Yoichi Usuiusui-yoichi@se.kanazawa-u.ac.jp
1
School of Geoscience and Civil Engineering, College of Scienceand Engineering, Kanazawa University, Nu‑7 Kakuma, Kanazawa, Ishiakwa
920‑1192, Japan
2
Volcanoes and Earth’s Interior Research Center, Research Instituteof Marine Geodynamics, Japan Agency for Marine-Earth Scienceand Technology (JAMSTEC), 2‑15, Natsushima‑cho, Yokosuka, Kanagawa
237‑0061, Japan
3
Atmosphere and Ocean Research Institute, The University of Tokyo,
5‑1‑5 Kashiwanoha, Kashiwa, Chiba 277‑8564, Japanthat affects global climate, such as radiative transfer andprecipitation. Thus, past changes in ice nucleation activity associated with atmospheric K-feldspar could haveaffected global paleoclimate, but there is limited evidence for these scenarios. Pankhurst (2017) argued that
because granitic upper continental crust contains more
K-feldspar than basalt, there should be a Gyr scale secular increase in the dust K-feldspar content following thecontinental growth. Studying meteorite impact records,
Pankhurst et al. (2021) pointed out a positive correlation
between the K-feldspar content of the target rock and theassociated extinction intensity. They suggested that thiscorrelation reflects global cooling by K-feldspar- rich
impact ejecta. However, these hypotheses lack actual relics of eolian K-feldspar. Consequently, the proposed highice nucleation activity could not be tested.
Pelagic sediments are long-term archives of atmospheric dust including continental materials and volcanic ash (e.g. Rea 1994; Dunlea et al. 2015b). However,
little attention has been paid to the detailed feldspar© The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Usui and Yamazaki ﻿Progress in Earth and Planetary Science(2023) 10:47

mineralogy in marine sediments. Previous studies ofdust mineralogy have often focused on clay to decipher
continental climate and provenance (Singer 1984). Onlya few comprehensive studies have examined bulk sediments or materials treated by HCl (Heath 1969; Peterson and Goldberg 1962). These studies reported low
K-feldspar contents that show little temporal variation.
In contrast, a recent study of South Pacific pelagic clay
using strong chemical digestion to remove clay minerals showed a marked increase in K-feldspar in the
deeper (older) section (Usui et al. 2018). However, thechronology of the sediment and the possible origin of
the K-feldspars were not thoroughly discussed.
K-feldspar has also been reported from marine sediments as an authigenic phase, often associated withvolcanic materials (Bass 1976; Donnelly and Nalli
1973; Kelts and McKinzie 1976; Lancelot et al. 1972;
Matthews 1962; Mellis 1952; Stewart et al. 1973). If
one wants to use pelagic sediments as archives of past
atmospheric dust minerals, it is necessary to distinguish between eolian and authigenic grains. Empirically, authigenic K-feldspars are chemically pure andcontain negligible Na and Ca even formed within
marine sediments (e.g. Kastner and Siever 1979). They
often constitute only a small fraction of the sediments,
except some volcaniclastic sediments contain up to 60%
K-feldspar (Kelts and McKinzie 1976). Consequently,
stratigraphic variations of authigenic K-feldspar inpelagic sediments have not been studied. In pelagic
clay, it has been observed that the occurrence of K-feldspar correlates with that of clinoptilolite (Stonecipher
1976). This leads to the suggestion that K-feldspar may
form from the diagenesis of clinoptilolite as in saline
alkaline lakes (Kastner 1981). However, there is no
direct evidence for this reaction in pelagic clays, and it
is unclear if they contain authigenic K-feldspar.
Here we present the feldspar mineralogy of two piston cores (KR13-02 PC05 and MR14-02 PC11) collected around Minamitorishima Island, North Pacific.
Together with South Pacific data from International
Ocean Drilling Program (IODP) Site U1366, our results
indicate that K-feldspar is the dominant feldspar for
sediments older than Miocene–Oligocene. We discuss
the possible origin of the K-feldspars.

2 Materials
We studied two piston cores of pelagic red clay, MR14E02 PC11 (22° 59.02′ N, 154° 0.98′ E, 5647 m water
depth) and KR13-02 PC05 (21° 59.03′ N, 153° 56.35′ E,
5735 m water depth), obtained around MinamitorishimaIsland (Fig. 1). MR14-E02 PC11 was first dated by Os
isotope stratigraphy (Nozaki et al. 2019). Subsequent

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paleomagnetic studies indicate the low 187Os/188Os
signature around 34.4 Ma have been spread out after
deposition, resulting in a significant overestimation of
sedimentation rate (Usui and Yamazaki 2021). Corroborated with Os data and constraints from realistic sedimentation rates, they assign 35–38 Ma at 12.0 m and30.6–28.1 Ma at 9.0 m. Chronostratigraphy of KR13-02
PC05 was also estimated using Os isotope ratios (Ohtaet al. 2020). It has not been examined if the Os isotope
signature is affected by post depositional processes asin MR14-E02 PC11. Nonetheless, microfossil ages arebroadly consistent with Os isotope data. Therefore, wewill use Os isotope ages for sediments younger than
28 Ma, while we rely on magnetostratigraphy or microfossil age constraints before that time. Bulk chemicalcompositions of cores obtained in this region establishedregional chemostratigraphy, which divides sediments
into five units; unit I to unit V from the youngest to oldest (Tanaka et al. 2020a, b). The chemostratigraphy indicates that KR13-02 PC05 contains sediment much older
than MR14-E02 PC11 (Tanaka et al. 2020a). This is also
confirmed by our magnetostratigraphy measurements(Fig. 1b, c). The upper part of unit V sediment from different cores contains biogenic magnetite indicative ofhigh surface production (Usui et al. 2018; Yamazaki et al.
2020). Yamazaki et al. (2020) correlate this signature toa paleo-equatorial high-productivity region, ~ 70 Ma
for Minamitorishima. This interpretation indicatesthat KR13-02 PC05, 9.0 m, may be older than ~ 70 Ma.
Regional acoustic survey (Nakamura et al. 2016) and chemostratigraphic and magnetic correlations (Tanaka et al.
2020a; Yamazaki et al. 2020) indicate that the surface sediment in the studied cores may not be of zero age. The
Os isotope data (Nozaki et al. 2019; Ohta et al. 2020) suggests core top ages to be ~ 2 Ma at 0.014 m in MR14-E02
PC11 and ~ 11 Ma at 0.11 m in KR13-02 PC05. A hiatus
was reported at the boundary between unit I and unit
II in MR14-E02 PC11 (Nozaki et al. 2019). In summary,
MR14-E02 PC11 covers ~ 40 to 2 Ma, and KR13-02 PC05
~ 70 to 10 Ma.
In addition to the Minamitorishima cores, we reexamined the mineralogy of International Ocean Discovery
Program (IODP) Site U1366 in the South Pacific (Expedition 329 Scientists 2011). The sediments in IODP Site
U1366 are divided into two lithostratigraphic Units
I and II. Unit I is subdivided into three Subunits Ia, Ib,
and Ic. Feldspar in Unit Ia is mostly plagioclase. K-feldspar becomes enriched relative to plagioclase down-core(Usui et al. 2018). An age model based on cobalt flux and
Os isotope data (Dunlea et al. 2015a, 2021) suggests thatthe boundary between Unit Ia and Ib may be ~ 44 Ma,
and that for Unit Ib and Unit Ic is ~ 62 Ma.

Usui and Yamazaki ﻿Progress in Earth and Planetary Science(2023) 10:47

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(a)

576

LL44-GPC3

Minamitorishima

U1336

Fig. 1 a Map showing the positions and backtracks of the study site (Minamitorishima) and the sites mentioned in the text. Dots represent
paleopositions for every 5 Myr back to 70 Ma. Plate motion was calculated using GPlates version 2.3 (https://​www.​gplat​es.​org) with the rotation
model of (Müller et al. 2019). Correlation between b KR13-02 PC05 and c MR14-E02 PC11 by magnetic susceptibility (MS) and chemostratigraphy(Tanaka et al. 2020a). Magnetic susceptibility was measured with 7 cc cube specimens using Kappabridge (AGICO, KLY-4). Arrows indicate
age constraints obtained by ichthyolith stratigraphy (Ohta et al. 2020), Osmium isotopes stratigraphy (Nozaki et al. 2019; Ohta et al. 2020),
and magnetostratigraphy (Usui and Yamazaki 2021). lE stands for late Eocene, and eO for early Oligocene. Orange horizontal line in c indicatesa hiatus

3 Methods
3.1 Chemical digestion

We performed digestion by citrate-bicarbonate-dithionite (CBD) (Rea and Janecek 1981) and sodium pyrosulfate fusion to extract feldspar and quartz (Inoue et al.
2021; Kiely and Jackson 1964; Syers et al. 1968; Usui et al.
2018). Dry samples of ~ 1 g were first treated with CBD
to remove poorly crystalline Fe–Mn oxyhydroxides. The
residues were washed with purified water, freeze-dried,
and weighed. The oxyhydroxide-free residues were thensubjected to the sodium pyrosulfate fusion as follows.
First, the residues were fused with ­Na2S2O7 at 460 °C todecompose sheet silicates. Next, the fusions were treatedwith 3N HCl and washed with purified water. Then, theresidues were heated to 50 °C in 1 M NaOH overnight
to remove relict clays. The sodium pyrosulfate fusion
was repeated 2 or 3 times to ensure complete digestion. While the CBD and acetic acid treatments havebeen widely used to extract the eolian component frompelagic sediments, these methods were ineffective inremoving biogenic apatite (Usui and Yamazaki 2021) andauthigenic zeolite (Ziegler et al. 2007), even though thosephases sometimes account for over 30 vol% of the sediment in the studied region (Iijima et al. 2016; Ohta et al.
2020). Strong acid treatment should dissolve biogenic
apatite while preserving quartz, but it is not yet clear if
the pyrosulfate fusion would remove all clay minerals andzeolite and if it would attack feldspars. While we focus on
the pyrosulfate fusion residues, we also compare materials obtained by the CBD treatment and bulk, untreatedmaterials to examine the effect of pyrosulfate fusion. In
any case, the materials analyzed in this study should be
regarded as operationally defined components.
3.2 Mineralogy

We examined the mineralogy of the pyrosulfate fusion
residues by X-ray diffraction (XRD). XRD measurements
used a diffractometer (MiniFlex II, Rigaku) with Cu
Kα radiation. To check the potential effect of chemicaldigestion on the mineralogy, we also measured selected
samples in untreated conditions and after the CBD treatment. We estimate mineral proportions by the ReferenceUsui and Yamazaki ﻿Progress in Earth and Planetary Science(2023) 10:47

Intensity Ratio method with full pattern summation
using the software PowdR (Butler and Hillier 2021). We
used reference patterns of K-feldspar, plagioclase, quartz,
and clinoptilolite from the RockJock reference library(Eberl 2003). We also included glass, tephra, and obsidian reference patterns because some samples revealed abroad peak. We will call them collectively "glass"; however, it is introduced to fit the observed background of
the spectra, and not much interpretation should be madeon this phase. We also reexamine the XRD data fromIODP Site U1366C (Usui et al. 2018) using the same protocol. The fusion residues were also examined under ascanning electron microscope (SEM; JEOL JSM-6010V)
equipped with an energy-dispersive spectrometer EDS.

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3.3 Grain size

Grain size distributions were measured for the fusion
residues. The samples were first dispersed in a 0.2%
sodium hexametaphosphate and sonicated for 1 h. Thenthe dispersed sample was introduced into a laser diffraction particle size analyzer (Mastersizer 3000E, Malvern
Panalytical) with purified water.

4 Results
The CBD treatment reduces the sample weight by
10–20%. Sodium pyrosulfate fusion further reduces the
sample weight to 10–30% of the original weight.
The XRD spectrograms can be fitted with the selected
phases in various proportions (Figs. 2 and 5). The only

Fig. 2 XRD diffractograms of fusion residues and their fits of a MR14-E02 PC11, 2.67 m, b MR14-E02 PC11, 8.99 m, and c KR13-02 PC05, 8.01 m. Data
are on the modified 2θ angles calculated by the PowdR software. The quartz peak at 26.6° is out of bounds

Usui and Yamazaki ﻿Progress in Earth and Planetary Science(2023) 10:47

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Fig. 3 The effect of chemical treatments on the XRD diffractogram of a, b MR14-E02 PC11, 2.67 m, c, d MR14-E02 PC11, 8.99 m, and e, f KR13-02
PC05 7.99 m. Blue, green, and red lines show untreated samples, samples after CBD treatment, and samples after sodium pyrosulfate fusion
treatment. The spectra was aligned and normalized using the quartz peak at about 26.6°. Black vertical lines are diagnostic peak positions
for plagioclase (b), K-feldspar (d), and clinoptilolite (f)exceptions are KR13-02 PC05, 2.66 m, KR13-02 PC05,
2.79 m, and U1366C-2H-5W, 143 cm, which indicate thepresence of small amount of additional phases (Additional file 1). These samples are excluded from further
discussion. We cannot completely exclude the presenceof additional phases in the other samples, but the XRD
spectra suggest that those unspecified phases should beminor components. Large variations in K-feldspar content relative to plagioclase are also evident from spectra
for bulk sediments before chemical digestion (Fig. 3).
This result confirms that the digestion treatments are
not the primary reason for the variations in K-feldspar
content. SEM/EDS analyses confirmed the presence of
K-feldspar (K-rich Al silicate) as well as plagioclase (Narich Al silicate) and quartz (Fig. 4). K-feldspar appears as
isolated grains (Fig. 4b) and grains covered by rhombohedral crystals similar to authigenic K-feldspar (e.g. Worden
and Rushton 1992) (Fig. 4c). The resolution of the SEM
limits detailed observations to grains over a few microns,
which is larger than the median grain size. Both types of
K-feldspar appear to contain Na. However, each rhombohedral crystal is too small to be measured separately, andthe signature may reflect submicroscopic heterogeneity. The quartz grains are more surrounded compared to
the feldspars (Fig. 4e). Otherwise, there is no significant

difference in size or morphology between the minerals.
The mass fraction of fusion residue and mineral proportions within the residue show systematic stratigraphic
variations. In the Minamitorishima cores, the fractionof the residues is the smallest around 30–35 Ma (Fig. 5a,c). In terms of feldspar mineralogy, all cores are dominated by plagioclase in the shallow part, while K-feldspar
is enriched over plagioclase in the deep part. K-feldspar
enrichment occurs before ~ 28 to 39 Ma (including late
Eocene to early Oligocene in KR13-02 PC05) for Minamitorishima cores, while it occurs before ~ 44 Ma for
IODP site U1366 (Fig. 5b, d, f ).
Some intervals show signs of increased volcanic input,
but this does not cover the entire interval where K-feldspar is enriched. A few samples contain a high proportion of glass within the fusion residue (Fig. 5b, d). These
intervals, i.e. ~ 6 m in MR14-E02 PC11 and ~ 3 m inKR13-02 PC05, correspond to the so-called "extremely
REY-rich mud" interval (Iijima et al. 2016), where the biogenic apatite content reaches ~ 30 wt%. Samples below
8.1 m in KR13-02 PC05 contain significant clinoptilolite(Fig. 5d), a diagenetic mineral likely derived from volcanic glass (Lisitzina and Butuzova 1982; Petzing and
Chester 1979). In addition, it should be noted that somevolcanic glass and zeolite are likely to be removed during

Usui and Yamazaki ﻿Progress in Earth and Planetary Science(2023) 10:47

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Fig. 4 SEM images of KR13-02 PC05, 5.04 m. a Image (left) and element map (right) of grains. Green shows K, and red shows Na. b An isolated grainof K-feldspar together with the EDS spectra. c A grain covered by rhombohedral crystals. d Plagioclase. e Quartz

Usui and Yamazaki ﻿Progress in Earth and Planetary Science(2023) 10:47

chemical treatment. In particular, phillipsite has beenreported to constitute up to 30% of bulk samples at 1.6–
2.0 m in KR13-02 PC05 (Ohta et al. 2016, 2020), but it
is not detected in our digested samples. However, thesevariations are not in phase with the consistently highK-feldspar fraction below 4 m in KR13-02 PC05 (Fig. 5b),suggesting that the K-feldspars are not direct volcanic
input.
We calculate the mineral content relative to the bulk
sediments using the mass fraction of fusion residues
and the mineral proportions within the residues. In the
Minamitorishima cores, the calculated K-feldspar content reaches 12 wt% of the bulk sediments in the deeper
stratigraphic level, while it is about 2 wt% in the shallower level (Fig. 6). At IODP Site U1366, the K-feldspar
content does not vary much because the low proportion of fusion residues in Unit Ib (Fig. 5e) (Usui et al.
2018) compensates for the high proportion of K-feldspar in the fusion residues.
Grain size distributions of the fusion residues generally
show the mode at 2 µm to 4 µm (Fig. 7). Smaller grain sizewith the mode at ~ 2 µm is observed in MR14-E02 PC11
below 11.99 m and in KR13-02 PC05 between 3.87 m to
7.05 m. These grain size variations resemble that of theeolian dust in the North Pacific (Janecek 1985; Janecek
and Rea 1983), where the grain size is finer between ~ 50
and ~ 30 Ma (Fig. 7e). The depth of the grain size change(11.99 m in MR14-E02 PC11) is different from that of
the mineralogical change (7 m). Thus, we interpret thesechanges reflect original grain size variations, even though
it may be possible that the grain sizes are reduced by thechemical digestion. Some samples show bimodal grain
size distribution with another maxima at ~ 20 µm, possibly reflecting sorting by strong bottom current (McCave
et al. 1995; Ohta et al. 2016).

5 Discussion
We discovered large variations in K-feldspar content
from North Pacific red clay. Together with the South
Pacific data (Usui et al. 2018), it appears that Pacific red
clay before ~ 30 to 40 Ma is enriched in K-feldspar relative to plagioclase. Moreover, in the North Pacific sites,
K-feldspar is a major component (> 10 wt%) of the bulk
sediments at that time. The most probable origin of the
K-feldspars seems to be either eolian or a replacement ofeolian plagioclase. Although K-feldspar may form after
volcanic materials (Kelts and McKinzie 1976; Lancelot
et al. 1972; Mellis 1952), they are not enough to explain
the K-feldspar enrichment. Clinoptilolite content is only
significant in the lowermost part of KR13-02 PC05 anddoes not correlate with K-feldspar content (Fig. 5d).
A modeling study using bulk chemistry also suggests

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the mass fraction of a volcanic component is limited
to be < 10 wt% between 4 and 7 m and ~ 20 wt% below
8 m (Tanaka et al. 2022). In contrast, K-feldspar content
already reaches ~ 10 wt% between 4 and 7 m in the core(Fig. 6b). These contrasting variations are difficult to
explain with K-feldspar formed after volcanic materials.
Eolian transport of the K-feldspars is a natural explanation for their variable abundance. Changes in either provenance or weathering conditions in the source area canresult in mineralogical changes. Indeed, mid- and lowlatitude sites in the North Pacific often show provenancechanges around 25–45 Ma due to a change in zonal wind
patterns or plate migration (Hyeong et al. 2016; Kyte
et al. 1993; Pettke et al. 2002; Ziegler and Murray 2007).
This timing coincides with the transition from K-feldspar-rich to plagioclase-rich sediments at the Minamitorishima sites. The similarity between grain size variationsof chemical digestion residues and previously reported
atmospheric dust records in the North Pacific (Janecek
1985; Janecek and Rea 1983) (Fig. 7e) further suggeststhat the K-feldspars are related to eolian dust. However,further mineral-specific measurements are needed toconclude this point. The variations in the eolian flux in
the North Pacific are less than a factor of 2 between 45
and 10 Ma (Janecek 1985; Janecek and Rea 1983). This ismuch smaller than the observed variations in K-feldspar
content, which reach a factor of 5. Thus, if the majorityof the K-feldspar is of eolian origin, the atmospheric dust
before ~ 28 to 39 Ma would have contained much more
K-feldspar than younger times. Because K-feldspar has
high ice nucleation potential and dominating the ice formation by silicate dust, our results suggest that clouds
over the Pacific before ~ 28 to 39 Ma tends to glaciate
more easily.
Alternatively to simple eolian transport of K-feldspar,
formation after eolian plagioclase may explain the observations. Although this reaction has not been considered
for pelagic clay, the microtexture (Fig. 4c) indicates thatat least some of the K-feldspars are formed through theauthigenic overgrowth. Formation via dissolution andprecipitation could convert plagioclase into isolated
K-feldspar grains (e.g. Putnis and John 2010) and explain
the grain size variations, even though we do not havepositive evidence for authigenic origin for such isolated
grains (Fig. 4b). Chemical composition data from isolated
K-feldspars (Fig. 4b) argue against a diagenetic origin,
as diagenetic K-feldspar has been reported to be chemically pure with minimal Na or Ca substitutions. However,
we cannot rule out possible averaging over submicroscopic heterogeneity in authigenic replacement. Detailed
microscopic work would clarify their origin (e.g. Lee andParsons 2003). It should be noted that the studied sites

Usui and Yamazaki ﻿Progress in Earth and Planetary Science(2023) 10:47

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Fig. 5 Stratigraphic variation of the fusion residues. a, c, e Mass fraction of the fusion residues for a MR14-E02 PC11, c KR13-02 PC05, and e IODP
Site U1366 (Usui et al. 2018). b, d, f Stack plots showing mineral proportions in the fusion residues of b MR14-E02 PC11, e KR13-02 PC05, and e IODP
Site U1366 (Usui et al. 2018). Ages are based on (1) Nozaki et al. (2019), (2) Usui and Yamazaki (2021), (3) Ohta et al. (2020), (4) Yamazaki et al. (2020),
and (5) Dunlea et al. (2015a). Qz: Quartz, Pl: Plagioclase, Kfs: K-feldspar, Cpt: Clinoptilolite. The chemostratigraphic units (Tanaka et al. 2020a) are
shown at the top of a, b, c, and d. For a color cord, see Fig. 1

are distributed within narrow paleolatitudes (Fig. 1a),where provenance changes around 25–45 Ma have beenreported (Hyeong et al. 2016; Kyte et al. 1993; Pettke et al.
2002; Ziegler and Murray 2007). Examination of samples
outside this latitudinal band would test if the K-feldspar
enrichment reflect time-dependent authigenic reactions
or eolian mineralogy reflecting dust provenance.

6 Conclusions
Pelagic clays around Minamitorishima Island, western
North Pacific, contain varying amounts of K-feldspar.
Before ~ 35 Ma it is > 10%, while after ~ 15 Ma it is 2–3%
(Fig. 6). IODP Site U1366C in the South Pacific also

shows K-feldspar enrichment over plagioclase before~ 44 Ma (Usui et al. 2018), but its concentration in thebulk sediments is not particularly high. Volcanic glassand zeolite show stratigraphic variations different from
K-feldspar. Previous geochemical data suggest that volcanic input is less than the mass fraction of K-feldspar.
Thus, we conclude that K-feldspar is not directly associated with volcanic input such as tephra. The K-feldspar
appears to contain Na, in contrast to nearly pure authigenic feldspars (Kastner and Siever 1979), although wecannot rule out nanoscale heterogeneity. Grain size variations of chemically separated feldspars and quartz are
similar to those of eolian dust in the North Pacific (Fig. 7)

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Fig. 6 Estimated mineral content in the bulk samples based on the XRD results and weight of fusion residues for a MR14-E02 PC11, b KR13-02
PC05, and c IODP Site U1366C. Ages are based on (1) Nozaki et al. (2019), (2) Usui and Yamazaki (2021), (3) Ohta et al. (2020), (4) Yamazaki et al.
(2020), and (5) Dunlea et al. (2015a). The chemostratigraphic units (Tanaka et al. 2020a) are shown at the top of a and b. For a color cord, see Fig. 1(Janecek 1985; Janecek and Rea 1983). These observations can be explained either by the eolian origin of the
K-feldspars or by authigenic formation replacing eolian
plagioclase. If they are eolian dust, then our data indicate that the past atmospheric dust had much higher
ice-nucleating activity than modern dust. Alternatively, if

the K-feldspar is of authigenic origin, the use of pelagic
clays as archives of past eolian dust minerals should bereconsidered. Further study in geographic distributionof K-feldspar enrichment is warranted to constrain past
atmospheric dust and potentially widespread redistribution of Na and K within pelagic sediments.

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Fig. 7 a, b Grain size distributions of selected fusion residues of a MR14-E01 PC11 and b KR13-02 PC05. Data are offset according to their depth,
and the depth of each sample is indicated on the right. c, d Depth variation of median grain size from c MR14-E02 PC11 and d KR13-02 PC05.
Samples with bimodal grain size distributions were not plotted. The paleomagnetic polarity zones of Usui and Yamazaki (2021) are shownon the right of c. Black indicates normal polarity, white indicates reversed polarity, and gray indicates a zone with an uncertain polarity pattern.
e Temporal variation of median grain size from LL44-GPC3 (Janecek and Rea 1983) and DSDP Site 576 (Janecek 1985). The geomagnetic polarity
timescale (GPTS) between 40 and 25 Ma (Ogg 2020) is shown on the right. The chemostratigraphic units (Tanaka et al. 2020a) are shownon the right of c and d. For a color cord, see Fig. 1

Usui and Yamazaki ﻿Progress in Earth and Planetary Science(2023) 10:47

Abbreviations
IODP	Integrated Ocean Drilling Program
ODP	Ocean Drilling Program
DSDP	Deep-Sea Derilling Program
CBD	Citrate-bicarbonate-dithionite
EOT	Eocene–Oligocene Transition
ITCZ	Intertoropical convergence zone
XRD	X-ray diffraction
REY	Rare-earth elements and yttrium

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​023-​00581-z.
Additional file 1. Samples with poorly fitted XRD diggractogram. A. KR1302 PC05, 2.66 m, showing unfitted peaks at around 19° and 38°. B. KR13-02
PC05, 2.79 m, showing ill-fitted peaks at around 13° and 27°. B. U1366C2H-5W, 143 cm, showing multiple peaks fitted with insufficient intensity
between 20°–35°. C. U1366C-2H-5W, 143 cm, showing peaks at around21°, 24°, and 29° fitted with insufficient intensity.
Acknowledgements
We thank Yasuhiro Takashimizu and Jyonaotaro Onodera for their instructions
and help in grain size measurements.
Author contributions
YU made contributions to conceptualization, methodology, formal analysis,
investigation, and wiring—original draft. TY made contributions to conceptualization, writing—review and editing.
Funding
This study was supported by JSPS KAKENHI Grant Nos. JP17H01361 andJP19H01997.
Availability of data and materials
The datasets supporting the conclusions of this article are available in the
Zenodos repository at https://​doi.​org/​10.​5281/​zenodo.​82106​25.

Declarations
Competing interests
The authors declare that they have no competing interests.
Received: 7 April 2023 Accepted: 3 August 2023

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