Paleoclimatology of Holocene and Pleistocene sediment from Vietnamese lakes

March 2016 – the first coring expedition in Biển Hồ

Our group of 7 (i.e. three professors and one graduate student from Vietnam National University Hanoi; three from Indiana, namely Anrdt, Minh and Jan who is applying for Bremen University) spent three days of hard work in and around Pleiku in the central highland of Vietnam. Dương Nguyễn-Thùy ‘s enthusiasm and insistence made it possible to overcome repeated resistance from local authorities who were suspicious about the involvement of foreigners in work concerning the precious freshwater supply of Pleiku. Equally important, Dương’s former adviser Prof. Phương Hòa Tạ’s excellent connections to some regional officials (in part his former students) provided critical support. Finally we broke the ice and received a research permit. Everywhere we go we are accompanied by a friendly local official to ensure a smooth operation.

Biển Hồ – hồ T’Nưng crater lake (maar) bathymetric map and sampled and trap set locations on March 2016 (Pleiku, Gia Lai).

The sensitive nature of the maar lakes (Biển Hồ – hồ T’Nưng – Đôi mắt Pleiku – Pleiku’s eye or Pleiku’s pearl) for Pleiku’s water supply prohibits most private boating. There is only one skipper authorized to operate his private boat. He is paid by the authorities to occasionally recover the bodies of drowned people. We rented his boat with a crew of 2. Only when we showed up the owner asked for $2000 rental fee. He had refused to spell out a price tag earlier. Luckily Professors Dương and Phương were able to bargain him down a little.

Topmost part of a sediment core from a lake.

There are three interconnected maar lakes (Biển Hồ) about 10 km north of Pleiku. The maximum water depth is about 25 m in the deepest maar. The second maar is about 22 m deep, and the third one is even shallower. The maars have slightly different water temperatures. We were able to run various transects with an echolot.  Hướng used his computer graphing skills to establish a preliminary colorized bathymetric map of all three lakes. There is a strong thermocline. Deeper water is oxygen-depleted. Our echolot “fish finder” did not indicate any fish near the bottom. The sediment is an extremely organic-rich, black sapropel with no trace of hydrogen sulfide. Upon warming of sediment, some small methane bubbles exsolved. Jan used the Gopro camera to take underwater movies during coring. The extremely water-rich sapropel remains very difficult to core with a traditional core catcher down to sediment depths of more than 5 m. It simply oozes back out of the core catcher when you try to recover a core. We estimate the water content at 5 m to 70 %. That bad! These maar sediments are very mineral-starved. Drilling of lateral holes in the upper part of the core liner helped a little, but still we could not recover a core longer than 0.5 m.

Comming soon...
Jan and Hướng with a “perfect” bouy designed for a sediment trap.

Most importantly, there is a bacterial mat covering the sediment/water interface. We could not see laminations below the mat through the plexiglass because the sediment smeared too much during its travel up and down the core liner. The sediment is way too unconsolidated to structurally withstand any transport. We are storing our short core upright in the house of a local fisherman for the next few months. With the help of the same fisherman and his son, we set a sediment trap with 3 collection chambers above a common mooring. We trained the fisherman to service the trap and collect samples for us in a 3-month rhythm.

The water level of the interconnected maars fluctuates by a few meters depending on the severity of the rainy season. The surrounding crater rims occasionally suffer wildfires. We saw plenty of charcoal and charred plant remains when walking around the maars. Jan and Anrdt are taking a sediment sample from a core catcher (stored inside a ca. 8 mm diameter plastic straw) with us to Indiana where we plan to use a plasma asher to see whether there is any rhythm in the inorganic content in the black sediment. We can also try to use epoxy impregnation and microscopy. Dương also has a short straw from the same core catcher and will do some mineralogical work in comparison with a basalt weathering profile that she sampled along a crater slope.

In the remaining days we are planning to lay the groundwork for a Vietnamese research proposal (officially spearheaded by Hướng) to attract funds for repeat coring and bolstering the Vietnamese efforts towards paleoclimatic maar research.

Working here is not without risks. The rented boat was not really safe. When carrying an ‘anchor stone’ from the shore onto the rickety ramp of the boat, Jan slipped, fell, and cut his chin on sharp metal. Dương inquired where a cosmetic surgeon was available. Jan was brought to a local Vietnamese Army Hospital (open to the public) and received 6 expert stitches at a very reasonable cost of only $35 (including medication). The same evening Anrdt suddenly felt extremely sick with tummy cramps. Apparently the dinner did not agree with his digestive system. He have not been in such pain for more than 30 years! Luckily, after a few agonizing hours all became normal again and Anrdt had a good sleep.

December 2016-January 2017 – the second coring expedition in Biển Hồ

We cored maar sediments near Pleiku, Vietnam. Our newly designed “autonomous sediment coring device with a pneumatically operated core catcher” worked well in the ca. 20-m deep maar lake. An alternate design using an extended set of metal rods pushing a coring device into the soft sediment also shows promise. We have half a dozen sediment cores that are about 50 to 70 cm in length. We are optimistic that a combination of both coring methods will yield several meters of sediment from the maar lake when we will return next time.

This time there was a distinct sediment layer on the surface that was fluffy and reddish-brown in color, very similar to the the clay that has been washing into the lake during the recent rainy season. We are now at the tail end of the wet season. We witnessed deep gullies that had been recently eroded along points of seasonal inflow along the perimeter of the maar lake. Jan often deployed the GoPro underwater-camera during coring. We never saw any fish, although we saw hobby fishermen with fishing poles catching some fish from shallow waters. The sediment cores often had quite clear water above the sediment, so we could very well observe the sediment/water interface. We never saw anything moving in the water and in the sediment. No worms, no nematodes.

As the GoPro descended, there was abundant light available for some time until there was a rapid transition into darkness before hitting the sediment surface. Arndt interpret this as a deep nepheloid layer that hasn’t had enough time to settle out. In contrast, in March 2016 we were able to see the sediment surface with the GoPro very well during coring. March is well into the dry season when there hasn’t been any significant for some time. Also, The March water level in the maar lake was significantly below the current water level.

Jan was also able to take a few short cores from ‘dry’ maars in the area, which are actually swampy at this time. One maar had a cement-like, very hard, dark layer at a depth of less than a meter below soft sediment. We suspect this to be a diagenetically hardened tephra layer. Dương has a sample and will find out using the mineralogical composition. A tephra layer should have a very low TOC content.  Jan could not penetrate the layer. In another swampy maar he was attacked by leeches and donated plenty of blood. Our next sampling trip to Pleiku will need to improve our core recovery from dry maar lakes. Maybe we really need vibracoring equipment.

All cores are now in Hanoi in the dark at room temperature in their core liners.

November 2017 – third coring expedition in Biển Hồ

We returned to Biển Hồ maar lake in November 2017 with with new home-built platform and sediment coring devices. We were delighted to welcome Dr. Antti Ojala from the Geological Survey of Finland as a new member to our team. Antti’s expertise in lake coring and core-to-core correlation using paleomagnetic sediment properties proved invaluable. Two days of sediment coring in Biển Hồ in November 2017 yielded 17 wonderful piston and gravity cores from up to 21 m water depth with up to 3.5 meter penetration into sediment. Our mobile coring platform and the newly designed coring devices performed better than expected.
In addition to coring in Biển Hồ maar lake, we also considered some shallower lakes and dry maars which can record bio-geochemical shifts relating to paleoenvironmental changes. We took short cores in Ia Bang lake and in the Plei Ốp swampy maar near Pleiku City.
The logistics worked out better than during previous coring expeditions, because our experienced team members know their duties and don’t require orders from the “boss”. We worked like a well-tuned professional coring team! See detail of our third coring expedition in Biển Hồ maar lake and core documentation and analyses in the laboratory at VNU in the session “I cored”.

Team members carry our coring platform from an assembly point near the shore to Biển Hồ maar lake in November 2017.
March 2018 – fourth coring expedition in Biển Hồ

As the fourth coring expedition in March 2018 , we were delighted to welcome Dr. Ingmar Unkel from Kiel University as a new member to our team. From Biển Hồ maar lake we retrieved 18 sediment cores with up to more than 5 meter penetration into sediment. Team members also did the very first freeze-coring ever in Biển Hồ.

Ingmar and Dương with our piston corer, March 2018.
October – November 2018 – fifth coring expedition in Biển Hồ

As the fifth coring expedition in October – November 2018, we were delighted to welcome Dr. Agnieszka Drobniak, Dr. Peter Sauer and Ph.D student Kelsey Doiron from Indiana University. Agnieszka led the water analysis team performing measurements along the water column in Bien Ho at three different locations. The most important aspect seems to be the depletion of dissolved elemental oxygen in Bien Ho water below 8 m depth. The coring team managed to piston-core 15 meters deep into Bien Ho sediment along repeat holes and offsets between the segments of the two piston core sequences. The recovered sediment may reach back 30,000 years. Our coring success is partially based on PVC casing that helped keeping the metal push rods straight and also let us re-enter the same hole repeatedly. The trick is to keep a good record of how deep we cored the last time, then going to the right depth, triggering the piston to lock, and then pushing the core liner further down. We also recovered sediment traps (Mar-Oct) and set new ones. Arndt was able to recover ‘grass with roots’ from about 4 to 5 m depth. The jury is still out whether ‘aquatic grass carpets’ ever existed. The lake level can go down by up to 5 m by the end of the dry season. A dive in March 2019 (i.e. at the end of the dry season) might offer a better answer. One complete set of cores was shipped to LacCore for analyses. Everybody is extremely happy.

The EOS team is now extremely efficient and experienced. With minor modifications, we are hopeful that next time (in 2020) we can possibly reach 30 m depth in sediment.

February 2020 – As a result of the coronavirus epidemic, the sixth coring expedition in Bien Ho had been delayed until March 2021

We had planned for field work in February 2020 to retrieve even deeper sediment than during previous campaigns. Several participants from the USA and Japan had purchased airline tickets to fly to Vietnam but were forced to cancel their travel owing to the Covid-19. We had to delay our coring until March 2021. The EOS team remains active in the Central Highlands with a land-based survey in  Biển Hồ and travel to Buôn Ma Thuột city to core in another lake.

Sediment coring in Ea Tyn Lake (February-March 2020)
Sediment coring in Ea Tyn Lake (March 2020)

In February-March 2020 (just before Covid-19 made international travel all but impossible), our coring team retrieved an 8.7 m long sediment core from Ea Tyn Lake covering the last 1200 years. The maximum coring depth was limited by the penetration capabilities of the available equipment and not by the solid-rock basement of the lake basin, which has not been reached yet. The unusually high sedimentation rate provides high temporal resolution for the last millennium. The new Ea Tyn Lake record, especially the summarizing XRF-based PC2 values are in good agreement with the tree-ring climate records of historically documented droughts.

March-April 2021, sixth coring expedition in Biển Hồ

Our latest coring expedition to Biển Hồ occurred in March-April 2021. We had improved our customized piston corer with larger-diameter metal rods and recovered parallel sediment cores with an overall length of up to 25 m, covering the last ca. 60,000 years. The fresh sediment cores were express-mailed to LacCore at the University of Minnesota in the USA for physical property logging, splitting, initial core description, photography, and high-resolution scanning for chemical composition. After the measurements at LacCore, the sediment cores were shipped to Indiana University’s cold storage.

Cores were subsampled and analyzed at Indiana University for sedimentology, loss-on-ignition, carbon stable isotope ratios, and radiocarbon and paleomagnetic dating by Hướng Nguyễn-Văn when visiting Arndt Schimmelmann’s laboratory from May to October 2021 as a Fulbright Scholar.

Funding:

1- NAFOSTED (2019-2021, code: 105.99-2018.316), PI: Hướng Nguyễn Văn
“Laminated Biển Hồ volcanic crater sediment in Central Highlands as a recorder of pre-instrumental history of monsoon and recent environmental recovery (Nghiên cứu lịch sử hoạt động gió mùa và biến đổi môi trường ghi nhận trong trầm tích hồ núi lửa Biển Hồ, vùng Tây Nguyên)”

2- VNU (2020-2021), PI: Thái Nguyễn-Đình
“Holocene paleoclimate recorded from lake sediment in the south of Central Highlands (Nghiên cứu cổ khí hậu giai đoạn Holocene khu vực Nam Tây Nguyên bằng dữ liệu trầm tích hồ)”

3-Fulbright Visiting Scholars Program (2021), PI: Hướng Nguyễn Văn, Host institution: Indiana University, USA “Multi-Proxy Reconstruction of Past Environmental and Climate Conditions in Vietnam’s Central Highlands Over the Last 50,000 Years Based on Crater Lake Sediment”

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  • Schimmelmann, A., Nguyễn-Văn, H., Nguyễn-Thuỳ, D., Schimmelmann, J.P., Ojala, A.E.K., Nguyễn, T.A.N., Quốc Trọng Đỗ-Trọng, Q., Nguyễn-Thùy, D., Tạ, H.P., Huỳnh-Kim, V., Phạm-Nữ, Q.N., Zolitschka, B., Unkel, I., 2018. Maar sediment in central Vietnam Highland near Pleiku: An archive of regional monsoon intensity? Maar2018, 7th International Maar Conference, 21-25 May, Olot, Spain, 2018. http://maar2018.com/ [Abstract PDF] [Poster PDF]
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A poster at Maar2018 – 7th International Maar Conference, 21-25 May, Olot, Spain, 2018.

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