The Layers of a Brownie (Lake): landscape manipulation and resulting geochemical changes in an urban waterbodyCore Lab Workshop (Geo 3880, spring 2007) class members: Jennifer Arp, Jason Dally, Kristin Frederick, Missy Gettel, Girard Goder, Andri Hanson, Jessica Heck, Farhana Jaafar Azuddin, Olufemi Kolawole, Kristine Powers, Megan Schreiber, Eric StewartInstructor: Amy Myrbo

STUDY SITE AND HISTORICAL CONTEXT

LITHOSTRATIGRAPHY

Unit 1: Reddish- to yellowish brown 0.5 cm-scale laminated diatomaceous clayey silt with calcite and aquatic organic matter. Base of unit is defined by first occurrence of fine laminae, above which a clastic-rich gray layer appears in all cores. In core 1A (shallow water), the time equivalent unit is a gray-brown calcareous sandy silt, often containing gastropod shells, interbedded with coarse aquatic and terrestrial plant remains.

Unit 2: Dark brown massive partially decomposed peat comprised of aquatic plant remains. Diatoms common; relatively minor clastics and trace carbonates.

CONCLUSIONSWe found several important differences between sediments deposited before and after the 1917 lake level lowering. Older sediments were evidently deposited under oxic, clearer water conditions, based on the absence of laminations and the abundance of aquatic plant fragments. Sediments that have accumulated since 1917, while the lake has been meromictic, are laminated and dominated by fine clastic material. We believe that the tan-gray layer represents a massive inwash of shallow-water sediments newly exposed to weathering by the 3 m drop in lake level; sedimentological evidence supports this interpretation.

Thanks to the Department of Geology and Geophysics for financial support, the Minnesota Historical Society for images, the Minneapolis Park and Recreation Board for permissions and encouragement, Ed Swain for the original study, and NSF via the LacCore Facility.

METHODSA transect of sediment cores from Brownie Lake was obtained using the MUCK (Multi-Use Coring Kit) surface coring system. Images and initial measurements were obtained following the procedures of Initial Core Description (ICD; Schnurrenberger et al., 2003). Whole cores were logged through the Geotek Multi-Sensor Core Logger (MSCL) for density, magnetic susceptibility, and non-contact resistivity. Cores were split into a working and archive half and the working half was cleaned by scraping the cut surface to expose fine sedimentary structures. Images of the working half were taken using a line-scan CCD camera, and split cores were run on the Geotek XYZ Logger for high-resolution magnetic susceptibility by point-sensor. Descriptions of sediment color and texture were made following methods found in Schnurrenberger et al., 2003. Smear slides were taken for petrographic microscopic examination (Kelts 2003). Samples from each core were taken at continuous 1cm intervals for loss on ignition (LOI; Heiri et al., 2001) to determine composition. Scanning electron microscopy of freeze-dried, powdered samples was conducted at the U of M Characterization Facility using the JEOL-6500 FEG-SEM. All cores are stored at 4 C in D-tubes in LacCore (Schnurrenberger et al 2001).

Heiri, O., A.F. Lotter, and G. Lemcke, 2001. Loss on ignition as a method for estimating organic and carbonate content in sediments: reproducibility and comparability of results. Journal of Paleolimnology 25: 101-110.

Kelts, K.R., 2003. Components in lake sediments: smear slide analysis. In B.L. Valero-Garces, ed., Limnogeology in Spain: A Tribute to Kerry Kelts. Madrid: CSIC.

Schnurrenberger, D.W., K.R. Kelts, T.C. Johnson, L.C.K. Shane, and E. Ito, 2001. National lacustrine core repository (LacCore). Journal of Paleolimnology 25: 123-127.

Schnurrenberger, D., J. Russell, and K. Kelts, 2003. Classification of lacustrine sediments based on sedimentary components. Journal of Paleolimnology 29: 141-154

Brownie

Cedar

Isles

Calhoun

Harriet

Loss on Ignition (LOI) data and smear slide data identify two distinct sedimentary deposition phases, which have been labeled Unit 1 and Unit 2. Unit 2, the older, stratigraphically lower phase is dominated by high levels of organic material. In core 3B, where the sediment deposition rate is lowest and the phases are most distinct, the organic rich layer averages roughly 41% of total dry weight. In general, the organic rich zone lacks laminae and has variable porosity levels.

The upper, more recent sediment is characterized by higher levels of carbonates, diatoms, and clastics. LOI data on core 3B shows the average organic component drops to roughly 17 dry weight percent, while inorganic and carbonate levels increase.

The transition from Unit 2 to Unit 1 is defined by the onset of laminations, which may or may not represent annual cycles of sedimentation. The lowermost portion of Unit 1 in core 3B shows a handful of fine laminae intermixed with organic rich layers. Further up Unit 1, a tannish-gray layer marks the beginning of permanent laminae in all three cores. Smear slide data show high levels of quartz, feldspar and other clastic components in the tannish-gray layer. This is verified by LOI data which show significantly higher levels of inorganics and carbonates. In core 3B, less than 5% organics are present in the tannish-gray layer, and similarly low levels are found in cores 1 and 2. This distinct band is not referred to by Swain (1984) or Tracey et al. (1996).

From the tannish-gray layer and above, laminae are distinct and consistent, indicating a lack of bioturbation and the onset of permanent bottom-water anoxia. Dating the onset of permanent bottom-water anoxia is difficult to accomplish. However, it may correspond with the completion of the 1917 canal linking Brownie Lake to Cedar Lake. Water depth dropped about 3 meters and the lake surface area was reduced by 56% (Swain, 1984). It was also around this time that storm sewer runoff was directed into the lake, which could cause some of the increased sedimentation rate seen in all three cores. The handful of fine laminae observed in core 3B below the boundary zone may represent temporary periods of anoxia as the lake transitioned from oxic to permanently anoxic bottom water.

The abundance of aquatic plant remains in the lower part of the core might be interpreted as evidence of shallower lake conditions in the past, if we did not know that the lake was actually deeper at that time. Higher water transparency (i.e., light penetration) could also have allowed higher vascular plant growth; Snelling (quoted in Shapiro and Pfannkuch 1973) noted that the lakes in the Minneapolis chain were transparent enough in the mid-19th century that one could see all the way to the bottom, even in the >25m deep lakes Harriet and Calhoun.

Shapiro, J., and H.-O. Pfannkuch, 1973. The Minneapolis Chain of Lakes: A study of urban drainage and its effects. 1971-1973. Interim Report 9, Limnological Research Center, University of Minnesota, Minneapolis.

Tracey, B., N. Lee, and V. Card, 1996. Sediment indicators of meromixis: comparison of laminations, diatoms, and sediment chemistry in Brownie Lake, Minneapolis, USA. Journal of Paleolimnology 15: 129-132.

CORING TRIP

NESW

UN

IT 1

UN

IT 2

flora

lfo

am 0%20%40%60%80%

100%

UN

IT 1

UN

IT 2

flora

lfo

am

UN

IT 1

SH

ALL

OW

WAT

ER E

QU

IVA

LEN

TU

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2flo

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foam

% Inorganic% CaCO3% Organic

Water chemistry profiles taken below the ice surface: Brownie Lake is meromictic (permanently stratified). Because its water column does not mix every year as most temperate lakes do, this leads to permanent oxygen depletion in the bottom waters and a buildup of dissolved solids (road salt ions and very high levels of Fe2+ and Mn2+).

Brownie

Brownie

Cedar Cedar

500 m

Railroad

Cedar Lake Pkwy bridge

By 1913 (map below, left) the Minneapolis urban area around Brownie Lake was densely developed. Airphotos (below, right ) show progressive development within the Brownie Lake watershed, as well as shoreline modification of both Brownie and Cedar lakes. Note the relatively low lake levels in 1937, presumably related to Dustbowl-era drought conditions.

In 1857 the area surrounding Brownie Lake was opened to settlers. Shortly thereafter, a local lawyer, W. W. McNair, purchased the land surrounding the lake and named it after the nickname for his daughter, Agnes.

In 1867 came the first of a series of major changes to the topography of Brownie Lake. That year a railroad embankment cut off part of the lake, reducing the area by 4.2 hectares and reducing the depth by up to 5%. The Minneapolis Park Board purchased the lake's catchment and decided to connect the Chain of Lakes (Brownie, Cedar, Lake of the Isles, and Calhoun) in 1907. In 1914 the dredging to connect Brownie and Cedar began, resulting in the lowering of the lake level by three meters.

The local storm sewer system was connected to the lake in 1920, an event having the capacity to dramatically affect the chemistry of the lake. On 29 July 1927, the lake was sampled by the Minnesota Department of Natural Resources (DNR) who described the vegetation as entirely cattails and white water lilies, relatively pollution intolerant species. This indicates that the lake remained relatively clean at that time. In 1933 the Park Board began the practice of pumping well water into Brownie Lake to maintain the level of the entire chain. In 1938 the canal between Brownie and Cedar was narrowed by the addition of fill. The construction of a series of east-west roads on the northern edge of the lake also disturbed the natural watershed. In 1894 Superior Avenue ran to the Northern tip of the Lake. The lake level was prone to fluctuation as shown by maps made between 1894 and 1914, and in 1917 the level dropped and in 1920 the road was repaved and widened by 1.2 meters and renamed Wayzata Boulevard. In 1948 Wayzata Boulevard became a four-lane highway. During the 1980's construction of Interstate 394 worked its way towards downtown from the west. The highway that parallels Wayzata Boulevard was opened in1991.

Minnesota began to use salt to de-ice winter highways in 1950. In 1964 the application rates of salt increased, and in 1968 peak application rates were reached.

Between 1953 and 1955 the Prudential Insurance building was constructed on the western shore which led to the landscaping of nearly fifty percent of the Brownie Lake shoreline. At the end of the construction the storm sewer draining the parking lot and the frontage road was connected to the lake. Prudential used water for cooling the building's air and computers. The building was sold to Target Corporation in 1994, at which time the use of cooling water was discontinued.

In 1957 a pump-line was built to transfer water into Brownie from a section of Bassett's Creek to maintain lake levels throughout the chain. From 1958 to 1965 an average of two billion liters per year were pumped. The water is chlorinated before it leaves the creek. In 1966 a pump was installed to pump Mississippi river water into the storm sewer system that empties into Basset's Creek, so that sufficient water would be available to pump out of the creek. Some effort was made in the 1970's to limit the pumping of Mississippi water to times when the total phosphorous concentrations in the River were less that of the Chain of Lakes. Swain, E.B., 1984. The paucity of blue-green algae in meromictic Brownie Lake: iron limitation or heavy metal toxicity? Ph.D. Thesis, University of Minnesota

Additional information provided by the Hennepin County Title and Abstract Office

Biological structures found in SEM (top, above) were identified as zooplankton filtering combs (feeding structures) by Bob Sterner, U of MN department of Ecology, Evolution, and Behavior. A Daphnia filtering comb is also shown (bottom) for comparison; the much smaller mesh size of the one found in Brownie Lake indicates that it comes from an organism filtering smaller (bacteria-sized) particles.

On Saturday, February 3rd, 2007, Amy Myrbo and her team of Paleoclimate, LRC core analysts, with 12 students of the Lake Sediment Analysis workshop began a research project to collect and describe the sedimentary deposits of Brownie Lake. It was an extremely cold winter morning. Temperatures ranged from -8 to -12 F with a windchill of -39 F! (See weather report at left.) We departed Pillsbury Hall of the University of Minnesota at 8:40 am and arrived at Brownie Lake at approximately 9:00 am. We offloaded the coring equipment and began the descent into the frozen lake. After three hurting hours of coring (which felt like five hours or more), we had succeeded in drilling and removing 4 cores from 3 different depths of the Lake. Some students we not used to the cold, and they took turns in sharing the warmth of the truck. For the brave students and analysts that stayed the entire coring process, we have to admit that after seeing the core samples in Lab the following Monday, all the frostbite, freezing eye lashes and sporadic sprints on the ice was well worth it!

48.8

cm

84 c

m9.

6 cm

14.5

cm

19 c

m50

.6 c

m58

cm

1937

1964

1991

Individuals of the calcifying green freshwater alga Phacotus, shown below (left) in petrographic smear slide, have two different appearances, one with a reddish band around the perimeter and one without. We hypothesise that we are seeing either the inside and outside view of the two hemispheric halves of the lorica (shell), or seeing one half and one whole lorica, although no petrographic description of this organism has been published to our knowledge. Interior and exterior views of the Phacotus lorica, both from Unit 1 of core 3B, are shown in scanning electron micrograph below, right.

light micrograph:note reddish band on individual on right

SEM: interior (left) and exterior views of Phacotus lorica. Euhedral particles in cup of shell on left may be diagenetic sulfide minerals.

This figure shows the relative sizes of Brownie and Cedar Lake before (left) and after the canal was completed from Cedar Lake (after Swain,

Northeast end of Brownie Lake (at lower right) looking east toward downtown Minneapolis (1949; MNHS)

Brownie Lake from west side looking southeast toward Cedar Lake (1890; cedarlakepark.org)

13.0 m water depth

RESULTS AND DISCUSSION

9.9 mwater depth

1.8 mwater depth

ostracode clastic mineral (amphibole?) iron oxide

clastic material (quartz)

diatoms

diatomsauthigenic calcite

(precipitated in the lake)

aquatic vascular plant material

"Zipper" joining two individuals of a Fragilaria diatom colony

Fragilaria

corroded calcite crystal

pine pollen

?detrital calcite crystal

Gastropod shell from core 1A shallow water facies. Shell is approximately 0.5 cm in diameter.

Magnetic susceiptibility (SI) Magnetic susceiptibility (SI)Magnetic susceiptibility (SI)

Wet bulk density (g/cm3)Wet bulk density (g/cm3)

INTRODUCTION

In the early morning of February 3rd, 2007 a team from the LRC (Limnological Research Center) set out into the coldest Minnesota weather in three years. Unfortunately for the chilly team members, this kind of weather helped the team's coring efforts. Collecting cores is easiest in the winter; the frozen lake can be walked on, and cores can be collected on the stable ice rather than a moving boat. Brownie Lake is located in an urban setting just outside of Minneapolis. It is the last of the Minnesota Chain of Lakes to the north and the smallest by surface area - more like a pond to most Minnesotans - but it is very deep for its size. The lake is surrounded on all sides by steep hills making the lake a small, sheltered basin. The team took careful steps down the steep hill to the lake's edge, and with augers in hand the seventeen-person team moved out on the ice.

We knew that the lake level of Brownie was lowered by about 3 m in 1917 when a canal was built connecting it to Cedar Lake. Our main question in this research project was: can we see this event in the sedimentary record? If so, how is it expressed? How does the depositional environment in the lake change when lake level and lake surface area are reduced?

Three hours and four cores later, the team had enough to analyze the sedimentation history of Brownie. Core 1A was taken near the shore in 1.8 m of water. It will represent the shallow-depth ecology and composition of the lake. Core 2A was taken on the slope (9.9 m water depth), southwest of 1A. We took two cores at the deep water (13 m) site: core 3B is the better of two. This core illustrates beautifully the laminations that occur after a curious thick gray band.

This layer was not obvious when the cores were extracted from the lake, but only when the cores were split and exposed to the air did the layering become apparent. Was this evidence of the lake level drop due to the construction of the canal? We set out to use macro- and microscopic lithological core description, geophysical measurements, and basic sediment analyses to answer these questions.

?

1917

2007

?1870

detrital calcite crystal

Cocconeis, an epiphytic (attached to a surface) diatom

The "tan-gray layer" that appears in deeper water cores 2A and 3B is comprised of sandy silt sized detrital siliciclastic and carbonate minerals, plus coarse organic matter particles. This evidence, along with the layer's location near the base of the laminated part of the core, supports the interpretation that it represents destabilized shallow-water sediment that was washed into the center of the basin when lake level was lowered (by connecting Brownie with Cedar Lake; see History section).

Sedimentary indicators of the lake lowering event

Conductivity (mS/cm2)

L A C C O R ENational LacustrineCore RepositoryNational LacustrineCore Repository

LacCore.org

Fragilaria colony

note paleoshoreline

Core components in petrographic smear slide and SEM(from core 3B except where noted)

This close-up (above, right) of a Fragilaria diatom colony shows the interlocking tooth-like structures that hold individuals together. Interlocked individuals are shown at left above, and a small colonial mass in light microscope view is shown at left.

Wet bulk density (g/cm3)

% Inorganic% CaCO3% Organic

% Inorganic% CaCO3% Organic

14.3m

0%20%40%60%80%

100% 0%20%40%60%80%

100%

Core transect locations. Contours are 3 m.