Text and images by Linda Welzenbach
The Kasten Corer is raised out of the water onto the back deck, lowered into its cradle and within minutes, a complex dance of core processing begins.
The processing and sampling of a soft sediment core is a little different from sampling rocks- hammers, chisels or rock saws have no use here! The sediment cores we are collecting consist of soft mud and often hold large rocks and pebbles, so handling them involves tools you wouldn’t expect- dry wall spatulas, kitchen sponges, water buckets, kitchen spoons and metal tools that aren’t too different from what you find in a dentist’s office.
Before the Kasten cores are carried into the lab, they get a quick sponge bath to minimize mess. Besides the obvious need to work in a clean environment, cleaning provides access to the screws holding the metal lids of the core barrel. When the lids are unscrewed and lifted off, the team gets its first look at their new treasure from the fathoms below.
Being entirely new to sediment coring, my first impression is that the cores look like a long box of melted milk chocolate. The top surface is fluffed up like meringue on top of a pie.
Out come the spatulas. Spatulas are used to scrape the mud off of the exposed core surface that was in contact with the barrel, as mud sometimes moves along the sides of the barrel during collection. We work diligently to make a smooth surface so that we can see the structure and texture of the mud within the core. Scraping seems like an easy task, but to make a surface perfectly smooth and not dig too deep or create new topography on the surface is harder than icing a cake. Once these cores are scraped smooth, we get our first peak at the story of Thwaites Glacier. Like pages in a book, the section of seafloor laying on the laboratory table allows THOR geologists to read the history of glacial advance and retreat.
The surface is now ready for the next step: the description. Describing the core involves a combination of observation and physical measurements. The description will direct us where to look for evidence of the timing and pattern of both past and present deglaciation. The description is like the plot of the story. It will be written from the various mud, silt, sand, and gravel-rich layers and relationships with any fossil remains. The detailed narrative will come from clues we can’t see immediately from the mud. Within each of those layers are specific chemical traces—or clues that can provide evidence for changes in the glacier and ocean environment over time.
From the base of the core to the top, the first physical measurement taken is the mud’s stiffness, also known as shear strength. The shear strength is a value measured from a device called a Torvane. Holding the Torvane by the spring-loaded disc, the instrument is rotated until the sediment is deformed by the flanges attached at the end of the rod. The value, read from the disc, tells geologists about the strength of the mud—which is a function of how compacted the sediments are and often related to amount of water present in the sediment pores. For example, the topmost sediments are the product of recent deposition of very fine suspended particles raining out of the water column onto the seafloor, and will contain a lot of water. Therefore, it will not be very stiff, resulting in a low Torvane value. Glacier till that was deposited under the weight of the glacier, however, will be stiff and strong with high Torvane measurements.
Next, we describe the color and size of the sediment. The color of mud is described by comparing it with a standard color catalog called the Munsell Soil Color Chart. Sediment comes in an amazing spectrum of colors, from yellow to red to green to grey and nearly black, with every shade and tone in between. By classifying the color, we can learn something about its components. Color can tell us something about the conditions that existed after the sediments were laid down. For example, mud that is dark gray to black suggests high organic content (think about what coal seams from dead forests look like) and little exposure to oxygen, preserving that organic matter. Lighter red-brown mud is more likely to have been exposed to an oxygen-rich environment. Greenish colors can indicate a concentration of photosynthesizing organisms deposited and preserved on the seafloor.
Dr. Becky Totten Minzoni, THOR’s core specialist from the University of Alabama, starts describing the sediment cores from the bottom. Working her way to the top at centimeter-scale intervals ensures that small variations in sediment types are not overlooked. In addition to color and grain size, she will note any structural patterns and textures she sees. These help to define layers, like pages in a book. When she’s done, there will be a systematic, detailed record of the various sediment types for the time span a core represents. The result is an overarching outline of the core that includes the chapters, paragraphs and perhaps some of the descriptive sentences that the team will use to tell the story of Thwaites Glacier.
The sediment grain size, measured against standardized scales, will tell us about the environment of deposition, and possibly specific events that occurred where the sediments were laid down on the seafloor. Seasonal, daily (tidal influence for example), and even single events can be identified from minute changes in grain size, microfossils, and structures within the sediment.
So what do the different kinds of sediments tell us?
Sediment types are defined by color, grainsize, fossils, and any structural features she may see. Unique combinations of these characteristics are used to delineate sedimentary units within the core.
Pebbles and cobbles are sometimes found among the muddy layers. Called ‘drop stones’ or Ice Rafted Debris (IRD), they fall to the sea floor from melting ice bergs. The stones are originally part of the glacier long before the ice enters the sea. When the ice moves offshore and melts, IRD falls through the water and drops into the soupy mud and silt below. Drop stones are an indicator of open marine water, where dirty ice bergs can drift and melt, but sometimes IRD can also show evidence for an ice shelf break-up event or melting from below.
Another thing Becky is looking for is evidence of meltwater from the sediment. When meltwater plumes are released from glaciers, fine silt is carried from the bottom of the glacier out to sea, and that fine, light material stays in suspension as the water travels. When the sediment settles out of suspension, a silt-rich layer is deposited on the seafloor. Silty meltwater deposits will be a major focus of the THOR group’s project, as they reconstruct history of past glacier instability and its causes.
Once the description is complete, sampling commences. Each sedimentary layer will be sampled to extract a long and detailed story. That story is told by chemical and biological proxies that can’t be seen but are present within the sediment and provide indirect measurements of past glacial conditions and ocean water temperature and salinity. Simply put, proxies tell us about the environmental conditions when the sediments were deposited. These include glacial setting, ocean characteristics, and the source of water within the sediments.
Analyzing the samples will take place in labs across the globe, with samples being sent to Alabama, Texas, the UK, and Virginia where THOR PI’s will dive into the detailed proxy measurements. Each scientist on the THOR team is an expert for a different proxy that will help reconstruct the history of Thwaites Glacier. THOR scientists then get together with their proxy datasets to write the story as a coherent progression of what happened to Thwaites Glacier through time.
Some of the most important analyses that will be conducted are those that tell time. Sediments deposited over the last hundred years or so can be dated from the natural activity of the radioactive element Lead (Pb) in the sediments. This “time” marker can only tell us about the recent past, no more than 150 years. To date farther back in time, we look for calcareous shells to carbon date. This will be the focus of Rachel Clark’s dissertation, who is working at the University of Houston with Dr. Julia Wellner, the lead THOR PI.
Microscopic plankton communities are a critically important part of the oceanographic history in and around Thwaites Glacier. Not only do the single-celled diatom phytoplankton (sea surface microalgae that rely on light) tell us about past ice shelves by their presence or absence, but they can be used to reconstruct sea ice conditions and surface temperature through time. The larger single-celled zooplankton, foraminifera, can tell us even more about ocean water masses and how they have changed over time. In particular, the THOR team will use calcareous foraminifera that are associated with the relatively warm Circumpolar Deep Water mass to investigate the influence of ocean changes on past stability (or instability) of Thwaites Glacier.
First we have to find these microscopic organisms. Becky and Ph.D. students Victoria Fitzgerald (UA) and Rachel Clark (UH) smear muddy sediment onto microscope slides to look for diatoms. They then sieve samples to remove all the mud and to look at the larger, sand-sized foraminifera under the microscope. Already excitement is in the air as the team finds communities of foraminifera that are found in modern settings where CDW is present!
One important task on this cruise is to extract the living benthic organisms from the seafloor surface and calibrate our understanding of modern communities associated with CDW. Becky will study these modern communities, and the chemical proxies will be extracted from the calcareous foraminifera by Dr. CD Hillenbrand at the British Antarctic Survey. Together, this information will help us interpret the proxies we extract from older sediment.
Ultimately, the sediment cores collected over the last two months will tell us how much and how fast the ice retreated from the ocean basin in ancient times, and also provide information about the recent changes in the last century. Because we have only been able to directly observe retreat using aerial surveys since 1947 and satellite imagery in the last 25 years, these cores are essential for extending the instrumental record and taking a longer look into past stability of the Thwaites Glacier system. Through a longer look into the past, combined with an understanding of oceanographic and bathymetric controls on glacier stability, we will have a better idea of what to expect in the future as CDW continues to impact the ice margin.