The Japanese Hirofumi Asahi from the Korea Polar Research Institute uses microfossils to tell about the changes in the Bering Sea and Arctic Ocean since millions of years
By Kirstin Werner
A micropaleontologic sample on a microscope tray. (photo: Hirofumi Asahi) |
Slowly the left hand pushes the tray from left to right. The eyes follow the tiny needle tip under the stereomicroscope. The right hand smoothly guides the needle in sinus curves over the black tray: up and down, up and down, up and down. Without stopping the needle drives the eyes along angular or softly rounded, yellow, black or transparent rock crumbs, along glassy pins and spiky brown particles. It also passes whitish grains sometimes lustreless, but sometimes as shiny as porcelain can be. They may look like small snails, or appear as tiny postal packages. On one of these white lustreless packages the needle stops. “I could never pick with a needle”, my colleague Hiro smiles, making me look up from microscope work. “I would not know how to hold it. My hands are too shivery for using a needle.” Among micropaleontologists there a those picking with a needle and those using a brush. Picking with the brush is safer as fragile calcite grains are not going to break. The needle has a steel handle that fits better in the hand but bears the risk of destroying the sometimes very thin-walled parcels already with the first tip.
Hirofumi Asahi from the Korea Polar Research Institute in Incheon
studies the history of the Arctic Ocean and the Bering Sea.
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Hirofumi Asahi uses a brush. For many years, the
Japanese has been working with the petite white chalky grains, called planktic
foraminifera. “I am fascinated by viewing
the ocean floor under the microscope”, he says. Planktic foraminifera are dust-sized,
single-celled organisms that float as zooplankton through the upper few hundred
meters of the oceans. After their life cycle ended, they sink to the ocean
floor. Their calcareous shells, the white lustreless packages, remain buried in
the seabed. Even after millions of years they can be found in deep layers
of the ocean floor. Micropaleontologists use these fossilized shells to learn
about the history of the oceans. “I actually chose foraminifera on purpose as I liked the idea to work
with microfossils that can also be applied to geochemistry. I am not that much
confident on my taxonomy skills. So I wanted to have more evidence by
additionally looking at their stable isotopes or magnesium to calcium ratios
which tell about the water conditions the they have lived in”, Hiro Asahi explains. During his
master program at the Japanese Kyushu University he taught himself how to
distinguish between different types of foraminifera. “My supervisor didn’t have much of an idea about
foraminifers so I had to go talk to some taxonomists. And I read lots of books
and guides about the different species. Tough way.” Foraminifera build their calcareous shell from the
chemical components of the surrounding water. Paleoceanographers like Hiro
Asahi who study the history of the oceans analyse these chemical components and
draw conclusions about the character of past ocean currents.
Eventually you make a cool story
Unlike most of his colleagues who investigate fossil
foraminifera shells uncovered from the ocean floor, Hirofumi Asahi initially worked
with those foraminifera that today live in the ocean. He studied the seasonal
differences of foraminifers in the Bering Sea and the central Pacific: “We collected them from the
upper water layers in large containers, so-called sediment traps, that were
anchored in the deep ocean.” During his dissertation at Kyushu University Asahi determined the monthly foraminifera abundance and species composition in
the sediment traps. In addition, he measured the ratio of oxygen isotopes in
their calcite shells. Different isotopes of oxygen in foraminiferal calcite provide
information on temperature and salinity of the surrounding water. As a second
step, Asahi related his results from foraminifera geochemistry to the measured
temperatures and salinities of the actual surrounding waters. For comparing
modern water conditions with the characteristics of living planktic foraminifera
is crucial for paleoceanographers. “First,
we need to find about the relationship of foraminifera and the ocean currents in
which they drift. Since the calcite of their shells is produced biologically,
the translation of their chemical properties into temperature and salinity
often is not straightforward. Our task is to find a systematic behind”, Asahi says. Only then fossilized foraminifers can be used to draw conclusions about
former seawater conditions. “Once you spent some time at the microscope it becomes really interesting. In samples from the
sediment traps you will always see a peak of foraminifer abundance in spring
and in fall. Which can be explained by the nutrient supply from the bottom
water. Kind of obvious to see that. But sometimes, you see some irregular
stuff, and if you then for example find a bloom on the satellite photo taken
from above your study region during exactly the time you ran your sediment trap
study, you are really getting into the thing. You are like, ‘Oh that could be a nutrient!’ Or ‘Some kind of
water mass stratification must have been strong’. Eventually you are able to
connect the things to each other and make a cool story out of it. That is the
thing I really like about my micropaleontology work!”
Professor Takahashi, I'll be good!
As for many scientists for Hiro Asahi the time after his PhD studies was not easy. For several years he worked at the University of Tokyo but poorly paid. The telephone call of his PhD supervisor Kozo Takahashi some time during fall 2007 was thus a real surprise. “He was looking for a candidate that could join his international ocean drilling expedition into the Bering Sea. He said, ‘Are you sure you can do some work?’ Because at that time I even started to regret myself becoming a scientist since I couldn’t find a proper job. He also said, ‘I am sure you are pretty much interested in going on that cruise but I am a bit concerned about the low publication record you have so far.’” The very few, much sought-after spaces on an IODP (short for International Ocean Drilling Program) expedition are reserved for those scientists who publish their results as quickly as possible. “He was right”, Hiro admits, “At that moment I didn’t have that much publications out but I really wanted to go, so I told him: ‘Prof. Takahashi, forget about my past, I will be good!’”
During the IODP cruise the geological
samples are directly viewed under the microscope. (photo: Hirofumi Asahi) |
The IODP expedition aboard the US research vessel JOIDES
Resolution into the Bering Sea has left a huge mark on Asahi’s scientific future: “The cruise
was a complete turning point. Only since then I actually consider myself
a real scientist. The quality of sediment cores was great, there were so many variations
in colour that already told us on board about the stories of the past. That was
really motivating.” In shifts, Asahi and his colleagues spent twelve hours
a day working on the vessel for about two months in summer 2009. “We really became friends aboard. It was like a snap. Every hour a new core section came up. The technicians opened the core by
splitting the plastic tubes with the fresh marine sediments inside into two
halves. Our job was to view the sediments and write down what we saw.” For every centimetre of the 745 meters of sediment Asahi and his teammates
noted the different colours and the grain sizes, the type of minerals and
sometimes even the smell of the freshly recovered material collected from the
seabed. “And then we waited again,
sometimes twenty minutes, sometimes an hour until the next core came on deck.”
2.5 million years old foraminifers
Silently the sand trickles from the glass vial through the fine-meshed sieve. The hands tap gently against the frame of the sieve to separate the fine from the coarser sand. From here the coarse fraction of the sieved sand is poured into a porcelain dish. The sand is then brushed into a small metal bucket. Metal clicks on metal when the sample is divided into halves by the sample splitter. Dividing the sample is done as long as it needs to acquire a countable but more important statistically significant amount of foraminifers in a sample, usually this is about at least three hundred specimens. The counted foraminifera are later extrapolated onto the total amount of foraminifers in the sample.
Since the 1980s, satellites record daily sea-ice conditions
in the
Arctic. The sea-ice decrease also impacts the global climate.
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Since the IODP expedition Asahi not only works with modern foraminifera but
uses his knowledge about them especially for his paleoceanographic work. “Even before the expedition I used to work with
geological material. But to be honest, it wasn’t very interesting to me just because there
was no foraminifera in the samples.” In contrast, the material from the Bering Sea contains well-preserved
foraminifer shells as old as 2.5 millions of years. “I personally believe that the Bering Strait had a great influence on
the Bering Sea history. For the same reason I am curious about what had
happened further north in the Arctic at the same time.” Changes in the
Arctic affect the global climate. Since the 1980s, the Arctic sea ice significantly
declined. Looking into the past of the Arctic may help to understand how the
Arctic Ocean will behave in the future.
“In order to do so we actually need another long and well-dated sediment core from
the central Arctic. I would so much like to be there when such a core is drilled
hopefully in the near future”, Hiro says.
“Age control in Arctic geological sediment cores is the biggest problem because
in the Arctic. Marine sediments often do not contain any foraminifer carbonate
material which can be used for determining the age of the sediments.” In
spring 2012, the now fourty-one year old Japanese moved to South Korea. First, he
worked at Pusan National University in the south of the country. In 2014, Asahi
moved to the Korea Polar Research Institute KOPRI in Incheon near Seoul. Here
he continues his work in the Bering Sea but also started to investigate
sediment cores from the central Arctic. “At
KOPRI an Arctic scientist faces great opportunities. I have access to many sediment
samples from the central Arctic. This kind of work I could never do in Japan since
we do not have such a nice Arctic research institute there.” There is a National
Institute of Polar Research in Japan but most of their effort is heading to
Antarctic. A new institute targeting the Arctic Ocean has
been established in Japan recently, yet they have no geology program included. Even
though Asahi sometimes feels like eventually returning to Japan, as the work can
be lonely at KOPRI. “I have no one to
really discuss my data. Since I am so specialized, nobody does something
similar.”
Cautiously the tiny tip of the needle taps and turns the
white calcite package. An opening on the right side of the package becomes
visible. Planktic foraminifera are growing chamber by chamber with the youngest
chamber leaving an opening, called the aperture. Amongst other taxonomic
identifiers the aperture helps in distinguishing between foraminifer species. “Far to the north, we do not have so many
different foraminifer species. The most common one is the left-coiling type of Neogloboquadrina
pachyderma. It loves the cool and salty waters”, Asahi explains. “These are present in countless numbers in
the Arctic and in the Bering Sea. But every now and then I also see other types
who like to have it a little warmer.” After so many years working with planktic
foraminifera Asahi still remains curious: “I
know that foraminifera cannot think. But I sometimes really wonder how they
manage to produce these fantastic shapes.”