Yu et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-020-00339-x(2020) 7:22

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open Access

Alkenone surface hydrographic changes ofthe subarctic Northwestern Pacific since thelast glacial: proxy limitations andimplications of non-thermal environmental
influences
Pai-Sen Yu1* , Chia-Ju Liao2, Min-Te Chen2,3*, Jian-Jun Zou3,4, Xuefa Shi3,4, A. A. Bosin5, Sergey A. Gorbarenko5 andYusuke Yokoyama6,7,8

Abstract
We investigated an alkenone-based sea surface temperature (SST) and the hydrographic change records of thesubarctic Northwestern (NW) Pacific from the last glacial to interglacial. The core we investigated is a piston core
(LV 63-41-2, 52.56° N, 160.00° E; water depth 1924 m) retrieved from the southern offshore east coast of theKamchatka Peninsula, which is a location of high sedimentation rate, with highly dynamic interactions with thecold/warm water masses of the Bering Sea/the NW Pacific. Based on our alkenone analysis with a previously wellestablished chronology of the core, we found high glacial C37:4 contents suggesting larger freshwater influences
prior to the last deglacial in approximately 27–16 ka BP. The most significant features of what we found arealkenone indicative of “warming” intervals with minimum alkenone productions that occurred prior to the stadial
Heinrich event 1 and the Younger Dryas. In contrast, for the interval corresponding to the Bølling–Allerød period,
our alkenone analysis shows relatively “colder” but maximum alkenone productions. We conclude that thisparticular subarctic alkenone SST proxy record is mainly masked by non-thermal environmental influences, such as
strong shifts of timing and duration of the sea ice retreat and/or salinity changes in surface water at this site, which
could cause changes in water stratification that affect nutrient supplies of the upper ocean that modulate growthdurations of phytoplankton/coccolithophore productions. Our studies suggest that this subarctic alkenone “SST”
proxy record is indicative of the changes of seasonality that control the timing and duration of the blooming
seasons of coccolithophores. The alkenone “SST” proxy is also dominantly driven by water stratification effects that,instead of SSTs, reflect most likely a combination of the following local to regional climate and ocean current
patterns: (1) the amount of meltwater inputs from high mountain glaciers at Kamchatka; (2) less saline, nutrient-rich
Alaskan Stream waters from the Cordilleran Ice Sheet in the Gulf of Alaska; (3) downwelling waters associated with(Continued on next page)* Correspondence: menardii@mail2000.com.tw; mtchen@mail.ntou.edu.tw
1
Taiwan Ocean Research Institute, National Applied Research Laboratories,
Kaohsiung 80143, Taiwan
2
Institute of Earth Sciences & Center of Excellence for the Oceans & Center of
Excellence for Ocean Engineering, National Taiwan Ocean University,
Keelung 20224, Taiwan
Full list of author information is available at the end of the article© The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
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Yu et al. Progress in Earth and Planetary Science(2020) 7:22

Page 2 of 13(Continued from previous page)the interactions between the southward Eastern Kamchatka Current and the spinning-up of the North PacificSubarctic Gyre; and (4) the strength of the Kuroshio Current since the last glacial.
Keywords: Last glacial, Stratification, Alkenone, Sea surface temperature, Northwestern Pacific, Subarctic

Introduction
The Bering Sea, one of the marginal seas in the Northwestern (NW) Pacific, is a dynamic area to global climate changes. The hydrographic system in the Bering
Sea is dominated by the cool water mass from the Arctic
Ocean and warm water mass from the northeast Pacific.
The Eastern Kamchatka Current (EKC), the major surface current in the west blank of the Bering Sea, tightly
links with the Bering Sea and the northeastern Pacificcirculations (Woodgate et al. 2005; Stabeno et al. 2005;
Panteleev et al. 2006) (Fig. 1). In the winter season, thestrong Aleutian Low and accompanying southwesterly
winds drive the EKC, greater extension of sea ice formation and cooling of surface waters from the Bering Sea
along the offshore of eastern Kamchatka Peninsula. The
southward EKC, which is together with the Alaskan
Stream (AS) and a branch of the Northwest Pacific Subarctic Gyre (NPSG), feeds into the Oyashio Current(OC) in the Subarctic NW Pacific. The OC reaches its
southernmost position with maximum strength in thewinter, in contrast to the stronger intensity of the subtropical Kuroshio Current during the summer season.
The modern winter surface circulation pattern in thesubarctic NW Pacific could be regarded as an analog inthe changes of the glacial oceanic circulation.
The paleoceanographic climate changes of the Bering
Sea and the subarctic NW Pacific over a millennial timescale are largely determined by changes in sea ice, surface circulation, and their interactions and feedbacks(Gorbarenko 1996; Katsuki and Takahashi 2005; Maxet al. 2012; Méheust et al. 2016). The mechanisms thatlink the climate, sea ice extent, surface ocean condition,
and associated temperature and productivity in theupper ocean of the Bering Sea and the subarctic NW
Pacific are often hard to disentangle due to the poor
preservation of carbonates, which makes it difficult touse foraminifer-based proxies (i.e., Mg/Ca, oxygen isotopes) extracting the precise hydrographic estimates withoxygen isotope stratigraphy. Previous investigations havebeen able to provide several alkenone proxy-based,
accelerator mass spectrometric (AMS) 14C-dated scenarios for interpreting such mechanisms largely driven by
sea ice changes of the subarctic NW Pacific from the lastglacial (Ternois et al. 2000; Harada et al. 2004; Seki et al.
2004a; Caissie et al. 2010; Max et al. 2012; Schlung et al.
2013; Gorbarenko et al. 2017). During the Heinrich
event 1 (HE 1) and Younger Dryas (YD), the seasonalsea ice has increased from the east toward the NW
Pacific and retreated to the east for the Bølling–Allerød(B/A) interstadial and early Holocene (Méheust et al.
2016, 2018). Such cases happen today when the maximum sea ice extent is not adjacent to the NW Pacific(Zhang et al. 2010). However, these alkenone proxybased results in the Bering Sea and the subarctic NW

Fig. 1 Map of site locations of the subarctic NW Pacific used in this study. A series of sediment cores along the western blank of Bering Sea, off
Kamchatka, and the Okhotsk Sea are used for extracting last glacial alkenone temperature variability of the subarctic NW Pacific. Modern annual
mean SSTs in the Northern Pacific are shown with contours using oceanographic data from AVHRR for the 1982–2014 period (Banzon et al. 2016).
Core LV 63-41-2 (52.56° N, 160.00° E; circled bullet symbol) was retrieved at a water depth of 1924 m from the continental slope off Kamchatka.
Core sites of these published alkenone SST records (Seki et al. 2004b; Max et al. 2012) are marked by yellow circle symbols. Surface circulation
patterns of the Northern Pacific are abbreviated as follows: EKC, East Kamchatka Current; OC, Oyashio Current; ANSC, Aleutian North Slope Current

Yu et al. Progress in Earth and Planetary Science(2020) 7:22

Pacific sediments existed controversial. While someareas illustrate abnormally warm with low alkenone
productivity in the cold period (Ishiwatari et al. 2001;
Seki et al. 2004b, 2007; Harada et al. 2006), others suggested that it remains under cold conditions (Max et al.
2012; Meyer et al. 2016). Questions regarding this climate abnormality in cold episodes include why alkenonechanges in these archives may result in such abnormalities and what reason was responsible for these low alkenone concentrations at high latitudes. The complicationsof the Bering Sea and the subarctic NW Pacific current
patterns with the high seasonality are related to changes
in nutrient supplies, ocean salinity, and freshwater inputs
from melting sea ice or mountain glaciers. More investigations are needed to bring in-depth understanding on
the uses of the alkenone proxies in high-latitude oceans(Rosell-Melé 1998; Sikes and Sicre 2002; Seki et al.
2004b, 2005; Harada et al. 2006; Popp et al. 2006; Caissie
et al. 2010; Max et al. 2012).
Apart from what we have from core LV 63-41-2(Gorbarenko et al. 2017, 2019), which was located in
the Bering Sea and the subarctic NW Pacific (Fig. 1),
here we present new results of the alkenone sea surface temperature (SST) analysis of LV 63-41-2.
Though many proxies measured from the core havebeen well-presented, this study focuses on (1) presenting alkenone SSTs, C37 alkenone concentration,
and C37:4 records based on a well-established age
model of the core (Gorbarenko et al. 2017, 2019); (2)investigating the magnitude and patterns of surface
ocean conditions and upper ocean stratification off
Kamchatka since the last glacial by interpreting thealkenone proxies and, more importantly, their limitations; and (3) advancing our understanding on any
plausible mechanisms of climate, sea ice, SST variations, and surface circulations among the subarctic
NW Pacific from the last glacial by integrating published alkenone records of a more regional scale.
Oceanographic settings

The core LV 63-41-2 (52.56° N, 160.00° E) was recoveredin 2013 by the Russian R/V Akademik M.A. Lavrentyev
during the Chinese–Russian Joint expedition (Fig. 1).
The 467 cm long core was retrieved from a location at
the continental slope off the Kamchatka Peninsula at awater depth of 1924 m, where there is a highly dynamic
oceanic front between the cold water mass from theBering Sea and the warm water mass from the NW
Pacific. Numerous investigations have pointed out
coccolithophore blooms occurring mainly in early spring
and fall at the Bering Sea and the adjacent subarctic
Pacific Ocean (Takahashi et al. 2002; Harada et al. 2003;
Seki et al. 2004b, 2007). The EKC from the Bering Sea
and the AS, which is one of the large-scale currents inPage 3 of 13

the subarctic Northern Pacific, brings relatively cold/
fresh water to the south and along with the AS flows
southeastwardly along the offshore of the eastern
Kamchatka Peninsula and is one of the major components of the OC (Favorite et al. 1976; Stabeno et al.
1999). The NPSG interacts with the EKC/AS/OC and, in
contrast, brings relatively warm/downwelling water tothe eastern margins of the Kamchatka Peninsula by
Ekman transports. During the wintertime, the Siberian
High/Aleutian Low system plays a crucial role in the sea
ice cover and the intensity of winter. An extension of sea
ice off the Kamchatka enforces the EKC pathway tomigrate far from the Kamchatka Peninsula (Zhang et al.
2010). The site of core LV 63-41-2, located at the southwesterly position of the modern EKC pathway, receives
most of the terrestrial materials (i.e., ice-rafted debris(IRD), higher magnetic susceptibility values) from themelting of the sea ice via the EKC. In the summer season, the East Asian monsoon conveys more heat andprecipitation to high latitudes. Fluctuations in freshwater
discharge in the NW subarctic Pacific, in turn, serve amajor controlling role for upper ocean stratification andsea ice formation. Therefore, the resulting hydrographic
changes in the upper water column at Site LV 63-41-2
would be sourced from (1) freshwater input effects fromthe melting of sea ice from the EKC, (2) west-flowing
waters of the AS, and (3) river runoff from theKamchatka Peninsula and/or local mountain glaciermeltwater inputs to Site LV 63-41-2. Therefore, the sea
ice cover, sea ice melting with riverine freshwater inputs,
the EKC/AS/OC, and possibly, the NPSG spinning-up,
all have important roles on the upper ocean hydrographies at Site LV 63-41-2.

Methods/Experimental
Chronologies

The sediment of core LV 63-41-2 is a high-quality core ideal
for paleoceanographic studies, as the core is mainly composed of calcareous ooze with few IRD layers (Gorbarenko
et al. 2017). The chronology of the core has been wellestablished based on AMS 14C radiocarbon dates and stable
isotope stratigraphy of benthic foraminifera, as well as
graphic correlation to non-destructive records (i.e., magnetic
properties, color reflectance b*, and X-ray fluorescence corescanning) of a nearby core SO 201-2-12 KL and an absolute
dated δ18O archives of Hulu/Dongge record (Gorbarenko
et al. 2017). In this study, we also present an alternative
chronology by adding four more AMS 14C dates that havenot been previously used but give new age interpretation inparticular for the interval of late HE 1 to B/A and back to ~4 ka (Table 1; Fig. 2). The purpose of presenting two sets ofchronologies is to address any possible uncertainty in interpreting our data due to age models. The age differences inthese two chronologies are very small by comparing theYu et al. Progress in Earth and Planetary Science(2020) 7:22

Table 1 Radiocarbon ages and age controlling points used in
the Core LV 63-41-2 chronology
Depth
(cm)

Age (ka)Age (ka)

Gorbarenko et al. (2017)

This study

120a

9.12

9.12

126

9.51

127.5a

9.45

156a

10.6

159

11.08

11.08

167

11.31

11.31

234

13.08

13.08

251

13.42

13.42

273

13.79

298a
303

13.79
14.39

14.42

306a

14.42
14.62

337

15.42

15.42

348

16.16

16.16

357

16.51

16.51

379

17.56

17.56

393

18.12

18.12

402

18.60

18.60

405

18.78

18.78

423

19.25

19.25

434

19.54

19.54

a

AMS 14C dates

timing of millennial-scale climate events. In particular, thetiming of major millennial climate intervals such as stadial
HE 1, B/A interstadial, and YD is still clearly identified andwell-constrained irrespective of two chronologies (Fig. 2).
Continuous time scales between two successive age
control points were obtained by linear interpolations.
Sedimentation rates calculated by the use of the published age model (Gorbarenko et al. 2017) are approximately 40 cm/kyr at average. The samples in thealkenone analysis we presented were from an every 2 cm
interval, corresponding to a time resolution of ~ 35 to ~135 years (average 80 years) throughout this core. Therefore, these excellent materials could provide highquality, millennial-scale alkenone-based proxy records.
Alkenone concentration and SST estimates

We sampled the core for the analyses of alkenone biomarkers at every 2 cm interval and extracted alkenones
from ~ 3 g of freeze-dried and homogenized sediments
following a procedure described by Müller et al.
(1998). An internal standard n-C36H74 was added into
the extracted fraction F3 (alkenones and alkenoates)
prior to injection. The fraction containing alkenones

Page 4 of 13

was analyzed with a Hewlett Packard 5890 series N
gas chromatograph (GC) with a cool on-column injection, electron pressure control system, and flame
ionization detector. The GC is equipped with a 60 m
column (Chrompack CP-Sil5 CB, 0.25 mm × 0.25 μm).
Samples were then dissolved in a hexane solution. The
flow velocity of a carrier gas Helium was maintained
at 30 cm/s. For the analysis of the F3 fraction, the oventemperature was programmed to reach 70–290 °C at
20 °C/min, 290–310 °C at a speed of 0.5 °C/min, andthen isothermal at 310 °C maintained for more than
30 min. Quantification of the alkenones was further
identified by comparing the retention times with thesynthetic standards (adopted by Prof. M. Yamamoto at
Hokkaido University, Japan). The alkenone unsaturation index Uk’37 and its SST estimate were calculatedusing a global core-top calibration (60° N–60° S; Uk’37= 0.033 × T + 0.044) developed by Müller et al.
(1998). The mean standard error of the estimatedalkenone-SST is approximately 1 °C (Conte et al.
2006). The alkenone-derived temperature estimate is
identical to those based on Emiliania huxleyi culturebased calibrations from Prahl et al. (1988; Uk’37 =0.034 × T + 0.039). Meanwhile, in order to understandthe applicability of Uk37 and Uk’37 calibrations on thesubarctic NW Pacific core LV 63-41-2, here, all calculated Uk37 values were also converted into SST estimates by using calibrations from the continuous
culture of E. huxleyi (Prahl et al. 1988; UK37 = 0.040 ×
T − 0.104). The original Uk37 index, which incorporates the C37:4 compound, gives some benefits compared with Uk’37. More C37:4 contents have been foundfrom environments of low temperatures (Rosell-Meléet al. 1994) and/or low salinity waters (Harada et al.
2003). Higher C37:4 values of LV 63-41-2 would reflectsources of freshwater inputs that could be mainly fromsea ice melting and increased supply of freshwater/river runoff from the melting of mountain glaciers at
the Kamchatka Peninsula. Therefore, the calculated
C37 alkenone concentration and C37:4 contents areused in this study as proxies for alkenone productivity
and freshwater inputs, respectively.

Results
Alkenone temperatures and concentrations

We have generated millennial-scale alkenone-derived
temperature and its related biomarker signals, including
C37 alkenone productivity and C37:4-based estimates on
relative changes in the freshwater contribution, based on
samples from core LV 63-41-2. The alkenone SST estimates at core-top samples are in good agreement withmodern summer SST (9.3 °C in July; Locarnini et al.
2010). The overall alkenone SSTs range very largely
from 1.8 to 17.9 °C (with a mean of 8.8 °C ± 3.1 °C) andYu et al. Progress in Earth and Planetary Science(2020) 7:22

Page 5 of 13

Fig. 2 Downcore LV 63-41-2 alkenone proxy-based estimates since the last glacial. a Uk’37-SST, b alkenone concentration (or total C37), and c C37:4
freshwater index, all are plotted against age and compared to d published downcore δ18O stratigraphy and chronology (Gorbarenko et al. 2017;
this study). Records applied in different chronologies from Gorbarenko et al. (2017, 2019), and this study is indicated by blue and gray,respectively. Age differences between those two chronologies are minor on late HE 1–early B/A and late–middle Holocene, except for late
Holocene. Shaded intervals indicate LGM, stadial HE 1, B/A (sub)-interstadials, and YD. Modern summer and winter SSTs are indicated, respectively,by red and blue arrows on a. Also, ~ 9–8 ka events mentioned in this text are also shaded in c. Prior to HE 1, our mean alkenone concentration(0.038 ± 0.008 μg/g) and detection limit of measuring the alkenone concentration (0.02 μg/g; Seki et al. 2004a) are indicated, respectively, by ablack arrow and dashed line in bshow a large cooling of approximately 16 °C from thelast glacial maximum (LGM) to the onset of stadial HE 1
(Fig. 2a). It is worth noting that LGM SSTs (~ 15 °C on
average; 19–23 ka) are warmer than the present (~ 8.4°C in the core-top sample). Besides, large millennialscale SST fluctuations of approximately 5–7 °C, particularly in the LGM-HE 1 interval, are observed, which isconsidered moderate when compared with the modern
SST seasonality of approximately 10–11 °C at this site
(Locarnini et al. 2010). The millennial-scale SST fluctuations are muted in the alkenone concentrations high andwarming intervals B/A and Holocene. Furthermore, arising of alkenone SSTs from the early Holocene toward
the present ranges from ~ 7.5 to 10 °C, which is close to
the modern observed SST from June to October (Locarnini et al. 2010).
Changes in downcore C37 alkenone contents exhibit
greater values with more significant fluctuations (>0.2 μg/g) during the B/A interstadial (~ 0.06–0.63 μg/g)
and early Holocene (~ 0.15–0.40 μg/g) compared withother time intervals (Fig. 2b). Noticeable alkenone

concentration lows (~ 0.038–0.054 μg/g) are also
observed prior to 15 ka (LGM) and from 12.8 to 11.8 ka(YD). Noticeable millennial-scale alkenone concentration
highs that may link with the changes of alkenone productivity in the B/A are clearly observed here. The millennial alkenone concentration highs are consistent withthree “sub-interstadials” productivity highs based onother proxies measured from the same core (Gorbarenko
et al. 2017), suggesting that the productivity changes in
the B/A are proxy independent. Our alkenone concentrations reached maxima abruptly and maintained forapproximately 1–2 kyr by the end of the YD, and after
the maxima, decreased gradually toward a minimum
value of approximately below 0.1 μg/g from the early tolate Holocene, with two steps of slightly back to higher
values at approximately 9–8 ka (within the uncertainties
of our age models).
The apparent alkenone-based “warming” at most partsof the LGM, HE 1, and YD in our core is inconsistent
with and even at opposite to previously reported values
for the global LGM and millennial-scale coolingYu et al. Progress in Earth and Planetary Science(2020) 7:22

scenarios. We further observed that the timing andduration of apparent warming coincided with the intervals of very low alkenone concentrations (C37 alkenones)at the YD period (~ 12.8–11.8 ka) and prior to 15 ka(Fig. 2a). The alkenone estimated warming with very low
concentrations is not likely due to the uncertainties ofage models (Gorbarenko et al. 2017). Rather than casting
doubt on the accuracy of the chronology and alkenone
analysis, here we interpret that those apparent warming
would probably be much relevant to various nonthermal effects on the alkenone proxies such as theshifting of the optimum growing seasons of coccolithophorids, upper ocean hydrographic and circulation
pattern changes, and freshwater effects from sea ice/mountain glacier melting (see the “Discussion” section).
Alkenone C37:4

Our measured C37:4 contents at core LV 63-41-2 exhibit
large variations (Fig. 2c). The C37:4 contents fluctuatewith relatively maximum values at intervals of late LGM
and of HE 1 to YD, whereas its minima (< 5%) mostly
appear throughout the whole Holocene with the exception of two abrupt spikes (> 7%) at approximately 9.0 ka
and 8.6–8.2 ka. We further observed high C37:4 contents
exist in not only the B/A but also the stadial HE 1/YD.
Notably, the high C37:4 content starts from the onset ofstadial HE 1 and reaches its relatively high value ofapproximately 10% at the end of HE 1. At the B/A interstadial, the C37:4 contents show millennial or higher frequency changes and, interestingly, sometimes antiphased with alkenone concentrations. The C37:4 contents
have gradually decreased from the mid-B/A and thenpersists relatively high frequent variations throughout
the YD. The C37:4 contents persist low but exhibit two
short-lived higher values at approximately 9–8 ka (within
the uncertainties of our age models) throughout the
Holocene.

Discussion
Glacial alkenone “warming”: proxy limitations

Prior to stadial HE 1 and YD, the anomalous surface
warming shown in our alkenone proxy estimates coincided with the lowest alkenone productions at Site LV
63-41-2 (Fig. 2a, b). The glacial surface warming is not
only confined to the southwestern position off Kamchatka, as reported from this site but also presented insemi-closed marginal seas around the NW Pacific andnortheastern Pacific (Barron et al. 2003; Max et al.
2012). Marine sedimentary archives from the Japan Sea
and Okhotsk Sea are also characterized by such anomalous warmer conditions during glacial periods (Seki et al.
2004b). One question regarding this climate “abnormality” in the last glacial is that any non-thermal factors
have been responsible for warm bias associated with low

Page 6 of 13

alkenone productions. Our LV 63-41-2 LGM SST (~ 15°C in average; 19–23 ka) is ~ 6–7 °C warmer than the
Holocene SST (~ 8.6 °C in average), showing an unusualsea surface warming off Kamchatka that is inconsistent
with the global LGM scenario (CLIMAP 1981; MARGO
Project Members 2009). Though the gradients of Uk’37SST calibrations are decreased at high- and lowtemperature extremes (e.g., Prahl and Wakeham 1987;
Müller et al. 1998), we considered that this would not
make our SST estimates drift to such anomalous warming for the LV 63-41-2 samples from the LGM and glacial periods. We noted that the alkenone productivity istemperature-dependent in that the high-latitude samples
located around the margins of sea ice covers have alkenone concentration lows (Seki et al. 2004b; Max et al.
2012). If the alkenone concentrations are close to thedetection limit of measuring the alkenone unsaturation
ratio, the SST estimates would be in large errors. Very
low alkenone concentrations with no SST estimations
prior to 15 ka have been reported in the NW Pacific(Ternois et al. 2000; Barron et al. 2003; Caissie et al.
2010; Seki et al. 2004a; Max et al. 2012). Moreover, Sekiet al. (2004a) reported that prior to 15 ka, alkenone SST
estimates with low alkenone concentrations down to approximately 0.02 μg/g may have brought with it large
uncertainties. In this study, LV 63-41-2 sediment samples have been processed using the resemble procedures
and the same type of GC machine as well as Seki et al.
(2004a). In order to clarify each alkenone, we processeda threshold of the integration height of the GC-FID
chromatogram as a similar signal-to-noise ratio in ouralkenone analyses. We also observed that our alkenone
concentration lows (0.038 ± 0.008 μg/g; Fig. 2b) prior to
HE 1 are twice that of the detection limit of measuring
the alkenone unsaturation ratio and the results from
Seki et al. (2004a). Therefore, the very low alkenone concentrations reported here are not due to any GC instrumental issues or analytical errors.
The LV 63-41-2 alkenone concentrations in the interval of the LGM to HE 1 bring great uncertainties in ouralkenone SST analysis. Though large uncertainties of
our alkenone SST estimates occur during very low concentration interval, large millennial-scale SST fluctuations of approximately 5–7 °C, particularly in the LGMHE 1 interval, are observed. The millennial-scale SST
fluctuations at LV 63-41-2 are muted in the alkenone
concentrations high and warming intervals B/A and
Holocene (Fig. 2a, b). Though the millennial-scale patterns of our alkenone SST estimates could be highly associated with large uncertainties in our analysis for the
low concentration samples, the long-term warming
trend from the onset of HE 1 to the Holocene shown in
our record is for predominating temperature driven.
Furthermore, in comparing the alkenone identification

Yu et al. Progress in Earth and Planetary Science(2020) 7:22

in our GC analyses on one low concentration sample(LGM) and one high concentration (B/A) with retention
times and an internal standard (Fig. 3a, b), we found thatthe cold indicator C37:3 content in the LGM sample
reaches a minimum (peak area of 10.4 at LGM example
and 127.9 at B/A example). The mean peak area of theC37:3 alkenone at LGM is at least 10 times as small as
those at the B/A. The concentration of the two alkenones determined by obtaining the small peak area
would bias Uk’37 higher and in turn give apparently
warmer SST estimation.
The changes of the timing and duration of the coccolithophore blooms in annual cycles would bias alkenone
SST estimates. Investigations have been conducted toobserve coccolithophore blooms occurring mainly inearly spring and fall at the Bering Sea and the adjacentsubarctic Pacific Ocean (Takahashi et al. 2002; Harada

Page 7 of 13

et al. 2003; Seki et al. 2004b, 2007). During the LGM,
spring sea ice cover would have significantly advanced
southward and further occupied around the offshore ofKamchatka and the Sea of Okhotsk, as revealed by theIP25 sea ice variability from the NW Pacific (Méheust
et al. 2016). Consistent with the results from a previous
investigation (Seki et al. 2004b) and new evidence of low
alkenone concentrations in LV 63-41-2 during the LGM
in this study, we infer that the greater expansion of thespring sea ice cover would restrict light availability andnutrient supply in the upper ocean. The more expanded
LGM sea ice cover would have shortened the durationsof warm seasons during which phytoplankton/coccolithophores have been growing in the Bering Sea andsubarctic NW Pacific. With respect to offshore Kamchatka, we speculate that a shift of the timing and duration on sea ice covers could cause changes in upper

Fig. 3 Example chromatograms of glacial alkenone warming and downcore alkenone-related variations since the last glacial. Representative GCFID chromatograms of alkenone-containing sediment extracts from examples of a LGM and b the B/A interstadial are illustrated. Alkenone peakassignation: IS/Internal standard, C37:4, C37:3, C37:2. c freshwater signals C37:4, d residuals in alkenone unsaturation index (Uk’37 - Uk37; black) andalkenone SSTs (red), and e alkenone unsaturation index Uk’37 (blue) and Uk37 (gray), all are plotted against age and compared with the fpublished downcore δ18O stratigraphy (Gorbarenko et al. 2017, 2019). Shaded intervals indicate LGM, stadial HE 1, B/A (sub)-interstadials, and YD

Yu et al. Progress in Earth and Planetary Science(2020) 7:22

water stratification and nutrient supply at our studied
site. This would also modulate the magnitude of phytoplankton and coccolithophore productions and, therefore, bias our alkenone SST estimates.
As noticeable C37:4 contents in LV 63-41-2 samples
were observed (Fig. 3c), alternatively, we have tested our
LV 63-41-2 alkenone SST estimates by calculating Uk37
and Uk’37 (Prahl et al. 1988; Müller et al. 1998) andfound that the differences between the Uk’37 and Uk37
(i.e., Uk’37 − Uk37) are very small (approximately 0.1–0.2,
corresponding to 2–5 °C) (Fig. 3d, e) that it does notchange the pattern of our alkenone SST estimates. These
results suggest that our alkenone SST estimation patternis independent from the uses of either Uk37 or Uk’37
equations. The existence of noticeable C37:4 contents in
LV 63-41-2 is intriguing (Fig. 3c), possibly revealing theincreases of freshwater input since the end of the LGM
and through the B/A and YD. These high C37:4 abundances would be possibly relevant to a large amount of
melting sea ice and freshwater pulses (Rosell-Melé 1998;
Sikes and Sicre 2002; Max et al. 2012; Meyer et al. 2016)
from the Bering Sea or off the Kamchatka entering and/or reaching Site LV 63-41-2. Furthermore, the AS that ischaracterized by warm and less saline water from thenortheast Pacific would be jointly responsible for highC37:4 contents (Caissie et al. 2010; Meyer et al. 2016;
Smirnova et al. 2015; Maier et al. 2018; Gong et al.
2019). Our alkenone-related results, which are consistent
with non-alkenone-based proxy data and conceptualmodels in the northeast Pacific, are support for nonthermal environmental influences off Kamchatka.
The timing of the increase of the LV 63-41-2 C37:4 contents differs from what has been reported from the Sea ofJapan and the Okhotsk Sea, which exhibit no remarkableincreases of freshwater supply during the LGM (Seki et al.
2004a). We hypothesize that, based on the modern ocean
current pattern of the Bering Sea and subarctic NW Pacific, increased LV 63-41-2 C37:4 contents may have been
either brought by more meltwaters from the nearby Kamchatka mountain glacier via the Kamchatka River or by
the relatively less saline water of the eastern branch of the
EKC from the AS and NPSG waters, which mixed withmeltwaters from the Cordilleran Ice Sheet (CIS) in the
Gulf of Alaska during the deglaciation. In this scenario,
the southward EKC bringing some less saline water mass
and/or floating icebergs would directly flow along the offshore of the Eastern Kamchatka Peninsula into the downstream region (south of the Cape Shipunsky, Kamchatka)
during from the LGM through the YD.
Implications on hydrographic changes: time-interval
scenarios

Our new alkenone proxy analysis based on the core LV
63-41-2 adds more information on understanding thePage 8 of 13

surface ocean hydrographic changes. Our alkenone analysis not only brings information on temperature evolution but also the changes in various non-thermal
environmental influences in the Bering Sea and subarctic
NW Pacific over time since the last glacial. In below, we
argue the most likely climatic and oceanographic scenarios from the last glacial to the Holocene, with more regional alkenone proxy data sets compiled from thesubarctic NW Pacific (the west blank of Bering Sea, off
Kamchatka, the Okhotsk Sea) (Max et al. 2012) (Fig. 4)
as follows:
The last glacial (~ 20–17.8 ka)

During the last glacial, the EKC, accompanied by icebergs and meltwater, flow along the Kamchatka Peninsula appears to be strengthened and extended further
south, as evidenced by the presence of high IRD abundances at the Bering Sea and the offshore Kamchatka(Gorbarenko 1996; Bigg et al. 2008). The higher magnetic susceptibility of LV 63-41-2 also suggests that
more terrestrial sediments were carried by the sea ice tothe offshore of the Kamchatka Peninsula at this time
(Gorbarenko 1996). During the last glacial, the Okhotsk
Sea has been mostly sea ice-covered, with less vegetation, as revealed by higher IRD contents and relatively
low pollen abundances (Gorbarenko et al. 2019). Furthermore, a study based on IP25, a sea ice indicator, alsosuggested a greater expansion of the spring sea ice coveraround the sea of Okhotsk and off Kamchatka (Méheust
et al. 2016, 2018). The presence of a more expanded
spring sea ice cover limits the nutrient supply, and thepenetration depth of light into the upper ocean, andthat, in turn, shortened the growing seasons of coccolithophore productions. A shift of the timing and duration on maximum phytoplankton productivity would
bias the alkenone temperature estimates to warmer seasons. It explains the surface “warming” in the Okhotsk
Sea (MD012412) (Seki et al. 2004b) and in the southern
offshore of the Kamchatka (LV 63-41-2) (Fig. 4). The
stability of the upper ocean stratification off Kamchatka
during the LGM may have been strengthened due tomeltwater inputs from long persistence of spring sea ice
cover and the enhanced icebergs-carried EKC. The
massive meltwater would decrease the surface water salinity to form a freshwater barrier layer that decreases
the air–sea heat exchange as well as the upwelling of
nutrients, and both serve as an explanation for the“warming” and low production of alkenone at this timeinterval.
The HE 1 (~ 17.8–14.7 ka)

Our LV 63-41-2 alkenone SSTs exhibits a gradual warming but large, millennial-to possibly centennial-scale
fluctuations (~ 5–7 °C) since the LGM to HE 1, thoughYu et al. Progress in Earth and Planetary Science(2020) 7:22

Page 9 of 13

Fig. 4 Last glacial subarctic NW Pacific alkenone-SST changes and its linkage with high-to-low latitude climate variability. a Northern Atlantic
climate variability represented by NGRIP ice core δ18O sequence (GICC05; North Greenland Ice Core Project members 2004); b western Bering Sea
alkenone-SST records: SO 201-2-101KL, SO 201-2-85KL, and SO 201-2-77KL (Max et al. 2012); c off Kamchatka alkenone-SST records: SO 201-2114KL, SO 201-2-12KL, and LV 63-41-2 (Max et al. 2012; This study); d the Okhotsk Sea alkenone-SST records: LV 29-114-3 and MD012412 (Maxet al. 2012); and e East Asian monsoon climate variability represented by the Hulu cave δ18O sequence (Wang et al. 2004). Shaded intervals
indicate LGM, HE 1, B/A, and YD. Arrow shows the 8.2 ka cooling event

the Okhotsk Sea record shows a prolonged “plateau-like”
warming till approximately 16.8 ka and then a large cooling (~ 5 °C) in the later interval of the HE 1 (Fig. 4). It is
likely that during the HE 1, the Bering Sea and NW subarctic Pacific were still covered with sea ice year round(Méheust et al. 2016); the rapid changes of our alkenone
SSTs and C37:4 freshwater contents during the HE 1
would suggest a highly dynamic, unstable sea ice systemwith more frequent meltwater pulses into the sea.
Hydrographic reconstructions of subsurface temperatures based on Mg/Ca, along with δ18O on foraminifera
Neogloboquadrina pachyderma (L) at a nearby core SO
201-2-12KL, further suggested that cold and fresh conditions would enhance upper ocean stratification during
the HE 1 (Riethdorf et al. 2013). Far from the northeast
Pacific, several non-alkenone proxy data and conceptualmodels address that freshwater inputs resulted in an enhanced ocean stratification during the deglacial warming(e.g., Caissie et al. 2010; Meyer et al. 2016; Smirnova
et al. 2015; Maier et al. 2018; Gong et al. 2019). These
above results of subarctic NW Pacific are coincident
with the continental runoff and intensified iceberg calving in the northeastern Pacific, associated with the beginning retreat of continental ice caps during the HE 1
(Gebhardt et al. 2008; Hendy and Cosma 2008; Taylor
et al. 2014). During the HE 1, less saline water would be
brought by the AS to the southern offshore of the Kamchatka due to the dynamics of the CIS in the northeast
Pacific (Davies et al. 2011; Taylor et al. 2014; Maier et al.
2018). We further speculate that more less saline andwarm water had been dynamically transported by the AS
and NPSG to the subarctic NW Pacific during stadial
HE 1. Moreover, more rapid spinning up of the NPSG
would cause coastal downwelling near the southern offshore of the Kamchatka. It could be expected that the
LV 63-41-2 would be more influenced by warm waterYu et al. Progress in Earth and Planetary Science(2020) 7:22

from the NPSG (MD012416; Gebhardt et al. 2008). The
millennial- to possibly centennial-scale SST fluctuationsof LV 63-41-2 during the HE 1 imply highly dynamic interactions between the AS and NPSG. At the end of theHE1, the surface hydrographies in either the offshore of
the Kamchatka or the south Okhotsk Sea have switched
to warming that coincides with the strengthened Asia
summer monsoons (Wang et al. 2004) and boreal highlatitude climate (North Greenland Ice Core Project
members 2004) (Fig. 4). The surface current pattern inthe offshore of the Kamchatka must have been changed
dramatically by the end of the HE 1 due to a significant
retreat of sea ice covers that favored high alkenone productivity at LV 63-41-2 and higher biogenic opal productivity at SO 201-2-12KL (Méheust et al. 2016).
The B/A (~ 14.7–12.8 ka)

Our alkenone SSTs during the B/A show a relative“cooling” in contrast to the warming in the Bering Sea
and the Okhotsk Sea (Fig. 4). An ice-free condition withhigher biogenic productivity has been widespread in thesubarctic NW Pacific during the B/A interstadial(Méheust et al. 2016). It is consistent with what has been
indicated by the IRD coarse fraction and chlorins (aproductivity proxy) measured from the same core (Gorbarenko et al. 2017), as well as with what has been inferred from diatom assemblages and the sea ice coverindex IP25 (Méheust et al. 2016, 2018). A similar featureof the deglacial meltwater input in the AS and elevatedbiogenic productivity from the northeast Pacific afterlate HE 1, which is coeval with a rapid rising of the sea
level, is also observed (Gebhardt et al. 2008; Davies et al.
2011; Taylor et al. 2014). The apparent “cooling” shownin our alkenone SST record is likely to be better explained by more prolonged growing seasons of coccolithophores, which have not been living very restrictedly
in peak summer as in the last glacial to the HE 1 whensea ice cover has been largely expanded. In this respect,
we argue that the apparent “cooling” during the B/A has
been also partially biased by the seasonality of coccolithophores growing in the subarctic. As the “cooling” isonly observed in our record but not in other records
from the Bering Sea (SO 201-2-77KL, -85KL, -101KL)
and the Okhotsk Sea (MD012412), we also argue thatthe local ocean current pattern changes adjacent to thesouthern offshore of the Kamchatka is responsible forthe cooling. For example, the relative cold and nutrientrich EKC water may have been blocked by the more
freshwater inputs from melting mountain glacier around
Capes Kronotsky and Shipunsky near Site LV 63-41-2
during the B/A. This hydrographic condition is supported by relatively high C37:4 contents measured fromour core (Fig. 3c). All instances indicate that the surfacehydrographies in the Bering Sea and the subarctic NW

Page 10 of 13

Pacific are highly spatially variable due to the interactions of meltwater from mountain glaciers and local
currents.
The YD (~ 12.8–11.8 ka)

Alkenone proxies indicate a cooling in most of the subarctic NW Pacific (the Bering Sea, Okhotsk Sea, andthe offshore of the northern Kamchatka) during the YD(Fig. 4). Sea ice cover would be extended to the western
Bering Sea and likely further to the southern offshoreof the Kamchatka during the YD (Méheust et al. 2016).
Our alkenone SST record, again, as during the last glacial and HE 1, shows apparent “warming” during the
YD. A similar warming condition revealed from the foraminifer and Mg/Ca temperatures at the center of the
NPSG showed a relatively well-stratified surface ocean
during the YD (MD012416; Gebhardt et al. 2008). We
attribute the YD “warming” to the similar mechanisms
in which the changes of growing seasons of coccolithophores, the effects of fresh/meltwater barrier-layer, andpossibly, more dynamic interactions between the AS
and NPSG would have jointly played roles. This interpretation is consistent with high magnetic susceptibility
and high IRD coarse fraction that are indicative of theflooding of more meltwater inferred from the same coreduring the interval of the YD (Gorbarenko et al. 2017).
The more stratified surface water hydrography during
the YD would limit the vertical transfer of nutrients
and, therefore, alkenone productions as well (Fig. 2b)
The Holocene

Alkenone proxy records compiled here indicate an
early Holocene (approximately 9 ka) SST maximum
near approximately the Bering Sea (SO 201-2-77KL,-85KL; Fig. 4). The timing of the SST maximum isclose to the local solar insolation maximum (at 65° N;
Laskar et al. 2004). While Bering Sea SSTs have been
decreasing gradually since the early Holocene, a rise in
our alkenone SSTs (~ 7.5 to 10 °C) is from the early
Holocene toward the present (Fig. 4). This is inaccordance with no IP25 biomarkers and high alkenone SSTs off Kamchatka (Max et al. 2012), revealinga long-term sea ice retreat with ice-free conditions at
our site in comparison with that prior to the Holocene. However, at sites near the southern offshore of
the Kamchatka (LV 63-41-2) and the Okhotsk Sea(MD012412), the early Holocene is characterized by acooling rather than warming (Fig. 4). The early Holocene cooling observed here is not disentangled fromthe well-known, shorted-lived 8.2 ka cooling (Chenget al. 2009; Liu et al. 2013) due to age model uncertainties. The early Holocene cooling well-expressed in
the offshore Kamchatka and Okhotsk Sea is likely attributed to the strengthened AS cold waters associatedYu et al. Progress in Earth and Planetary Science(2020) 7:22

with increased meltwaters and icebergs from thenortheastern Pacific and meltwater flood at the offshore southern Kamchatka, with the Bering Sea notaffected. Those cold, nutrient-rich AS waters may also
contribute to phytoplankton blooms with higher coccolithophores and diatom abundances off Kamchatka
through the end of the YD to the early Holocene(Méheust et al. 2016) (Fig. 2b). The gradual transformation into “warming” with low productivity since theearly Holocene in the offshore Kamchatka andOkhotsk Sea is probably driven and complicated byvarious mechanisms such as a weakened or northward
movement of the Siberia High, with increased Asiansummer monsoon precipitation, and these climatic
factors, along with possibly more moisture brought by
the oligotrophic Kuroshio (Harada et al. 2004), may allcontribute the late Holocene warming and low alkenone productivity, as we observed in this study.

Conclusions
In this study, we have documented the last glacial organic geochemical proxy changes (including Uk’37-SST,C37 alkenone concentration, and C37:4 freshwater index)
from the core LV 63-41-2 at the southern offshore of
the eastern Kamchatka. Our studies provide new insights
into the deglacial surface hydrographic variability thathas been governed by both local to global climatic andoceanographic processes in the Bering Sea and the subarctic NW Pacific. The alkenone proxies have limitations
in estimating SSTs for areas such as of this study thatare characterized by strong shifting of timing and duration of warm seasons in which coccolithophores preferto live, various inputs of larger amounts of fresh/meltwaters that cause changes in surface water stratifications,and complex ocean current patterns that result incoastal downwelling or upwelling. We conclude that the
paleoceanographic reconstruction of the Bering Sea andsubarctic NW Pacific is complicated by the drastic local
changes of seasonality that affects the productivity ofproxy organisms, the extents of sea ice and mountainglaciers with varied amount of meltwater inputs, and the
dynamics of open ocean gyres that changes. All of theabove local to regional non-thermal environmental influences would bias our alkenone SST estimates for global
climate variability related to insolation, ice volume, monsoons, the Kuroshio, and basin-scale overturning
circulations.
Abbreviations
AMS: Accelerator mass spectrometric; ANSC: Aleutian North Slope Current;
AS: Alaskan Stream; B/A: Bølling–Allerød; CIS: Cordilleran Ice Sheet; EKC: East
Kamchatka Current; GC: Gas chromatograph; GOA: Gulf of Alaska;
HE: Heinrich event; IRD: Ice-rafted debris; LGM: Last glacial maximum;
NPSG: North Pacific Subarctic Gyre; NW: Northwestern; OC: Oyashio Current;
SST: Sea surface temperature; YD: Younger Dryas

Page 11 of 13

Acknowledgements
This study was supported by grants from the Joint Projects of MOST
(Ministry of Science and Technology)/FEBRAS (Far East Branch of Russian
Academy of Sciences) to MTC. The authors would like to thank the crew on
the R/V Akademik M.A. Lavrentyev during the Joint Russian–Chinese
Expedition for their coring and curatorial assistance.
Authors’ contributions
Taiwan Ocean Research Institute, National Applied Research Laboratories,
Kaohsiung 80143, Taiwan. Pai-Sen Yu Institute of Earth Sciences & Center of
Excellence for the Oceans & Center of Excellence for Ocean Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan. Chia-Ju Liao & MinTe Chen Pilot National Laboratory for Marine Science and Technology, Qingdao 266061, China. Min-Te Chen, Jian-Jun Zou & Xuefa Shi First Institute of
Oceanography, State Oceanic Administration, Qingdao, China. Jian-Jun Zou &Xuefa Shi V.I. Il’ichev Pacific Oceanological Institute, Far-Eastern Division of
the Russian Academy of Sciences, Vladivostok, Russia. A. A. Bosin & Sergey A.
Gorbarenko Atmosphere and Ocean Research Institute and Department ofEarth and Planetary Sciences, University of Tokyo, Japan / Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology, Japan.
Yusuke Yokoyama MTC, JZ, XS, AAB, and SAG proposed the topic, conceived,and designed the study. PSY and MTC collaborated with the corresponding
author in the construction of manuscript. CJL carried out the experimental
study. YY analyzed four more AMS 14C dates. All authors helped in their interpretations. All authors read and approved the final manuscript.
Funding
This study was supported by grants from the Joint Projects of MOST
(Ministry of Science and Technology)/FEBRAS (Far East Branch of Russian
Academy of Sciences) to MTC. This work is also partly supported by the
National Program on Global Change and Air-Sea Interaction (grant no. GASIGEOGE-04) and the National Natural Science Foundation of China (grant nos.
41476056 and U1606401) to JJZ.
Availability of data and materials
The datasets used and/or analyzed during the current study are available
from the corresponding author on reasonable request.
Competing interests
The authors declare that they have no competing interest.
Author details
1
Taiwan Ocean Research Institute, National Applied Research Laboratories,
Kaohsiung 80143, Taiwan. 2Institute of Earth Sciences & Center of Excellence
for the Oceans & Center of Excellence for Ocean Engineering, National
Taiwan Ocean University, Keelung 20224, Taiwan. 3Pilot National Laboratory
for Marine Science and Technology, Qingdao 266061, China. 4First Instituteof Oceanography, State Oceanic Administration, Qingdao, China. 5V.I. Il’ichev
Pacific Oceanological Institute, Far-Eastern Division of the Russian Academyof Sciences, Vladivostok, Russia. 6Atmosphere and Ocean Research Institute,
University of Tokyo, Tokyo, Japan. 7Department of Earth and Planetary
Sciences, University of Tokyo, Tokyo, Japan. 8Institute of Biogeosciences,
Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan.
Received: 1 January 2020 Accepted: 22 May 2020

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