stable isotopes in profundallake sediments applications

Stable isotopes in profundal lake sediments:

applications and perspectives for reconstructing lake ecological development at secular scale.

Monica Tolotti

Department of Sustainable Agro-ecosystems and Bioresources, Research and Innovation Centre (CRI), Fondazione Edmund Mach (FEM),

S. Michele all’Adige E-mail: [email protected]

Università Milano Bicocca, May 21th 2018

1. Stable (and radioactive) isotopes: what, where, how

2. The paleolimnological approach and isotopes in sediment records

3. Examples of application (literature)

4. Future perspective and developments

Contents

What are isotopes?

Isotopes = variants of a chemical element, which

- differ in the neutron number- have the same atomic number (protons) but differ in atomic mass.

Isos (same) + topos (place) = they occupy the same position on the periodic table

Unstable isotopes = tend to decay to a stable form emitting energy or particles; the decay produces in turn stable or radiogenic isotopes.

Stable isotopes = not radioactive (12C, 13C). stability is related to proportion between protons and neutrons in the nucleus

339 natural isotopes for 80 elements, including all stable isotopes. 286 = primordial nuclides existing since the formation of the Solar System.

~ 3,300 artificially created nuclides (e.g. nuclear fission, particle accelerators)

e.g.: 12C, 13C, 14C → forms of C (atomic number = 6) with 6, 7, and 8 neutrons


The decay of unstable isotopes represents the basis for application in:

a) Radiometric dating: 14C, 210Pb, 137Csb) Oncology, nuclear medicine: 60Coc) Sterilization: 60Cod) Nuclear power production, nuclear weapons: 238U, 137Cs

Nuclear properties

Protons → positively charged, they repel each other

Neutrons → electrically neutral and stabilize the nucleus (by pushing protons slightly apart,they reduce the electrostatic repulsion between the protons, and exert anattractive nuclear force on each other and on protons)

Nuclear stability depends on:

a) number of protons → as the number of protons increases, so does the ra�o of neutrons to protons necessary to ensure a stable nucleus

b) evenness or oddness of its atomic number Z, neutron number N and, of their sum, the mass number A. The majority of stable nuclides have even numbers of Z, N, and A.

Oddness of both Z and N tends to lower the nuclear binding energy → to make nuclei less stable. Unstable isotopes decay in different ways, emitting energy and particles.

Chemical and molecular properties

• different isotopes exhibit nearly identical chemical behaviouras the chemical behaviour of an atom is largely determined by its electronic structure

protium deuterium tritium

→ Isotopic fractionation = change of an isotopic ratio due to physical and chemical processes

a) Equilibrium fractionation

b) Kinetic fractionation

Quantitatively described by the Fractionation factor a = ratio (substrate)/ratio (product) (D!)

• lighter isotopes tend to react faster than heavy isotopes of the same element andbecome selected by the reactions.

The selection of lighter isotopes ismost pronounced for smalleratoms, where mass difference aregreatest in proportion.

The delta notation

Each material has a characteristic ratio of stable or radioactive isotopes = isotopic signature, or fingerprint

A relative measure = delta (d) notation, is used for isotopic signature, instead of an aboslute scale:

13C/12C = isotopic ratio = R

std = standard, conventinally established reference material

(other standard substances are used to verify the accuracy of mass spectroscopy,which is used to detrmine the isotope ratio of a substance. e.g. aniline for N)

( (

ElementStable

IsotopesAverage natural abundance (%)

Standard ratios International Standard

Hydrogen1H 2H

99.985 0.015

2H/1H = 0.000316

SMOW (Standard Mean Ocean Water)

Carbon12C 13C

98.892 1.108

13C/12C = 0.0112372

PDB (Pee Dee Belemnite)

Fossil Calcium Carbonate

Nitrogen14N 15N

99.63370.3663

15N/14N = 0.007353

AIR (air nitrogen)

Oxygen16O 18O

99.75870.2039

18O/16O = 0.0039948

SMOW (Standard Mean Ocean Water)

Sulfur32S 34S

95.02 4.22

34S/32S = 0.0450045

CDT (Canyon Diablo Troilite)

Isotopic natural ratios and standards

Pee Dee Belemnite = carbonatic rock, originating from the fossil cephaolopode Belemnitella americana,from the Peedee Formation in USA, which had an anomalously high 13C/12C ratio (0.0112372)


Stable isotope ratio analysis (SIRA)

Light elements (H, C, N, O, S) → Isotopic Ratio Mass Spectrometry (IRMS)measures ionic forms after transformation of substances in pure gases; usually combined with an elemental analyser (for element absolute amount determination)

Isotope lab at FEM

Drying → (acidifica�on) → pyrolisis at 1000°C → dehydra�on and purifica�on of CO, CO2 H2 , NOx SO2→ ioniza�on (ion source) → ion accelera�on and mass detec�on → ion detector

Heavy elements (Pb, Sr) → Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) were samples are separated in atoms and ions.

ion source

accelerator(magnete)

detector

Equilibrium fractionation (Oxygen)

H216O molecules evaporate faster → water vapor is depleted in 18O (lighter)

Challenge: years (up to 10!) of monthly isotopic measurements of precipitations are necessary for a reliable signature of d18O in precipitation over a certain region.

Equilibrium fractionations depends ontemperature

→ d18O of atmospheric precipitation at anylocation depends on air mass trajectories,seasonal changes in air temperature, source ofmoisture (ocean, forest etc.).

Kinetic fractionation

Reactions that are offset of thermodynamic equilibrium (e.g. rapid freezing, high precipitation)

Biologicla reactions (e.g. photosynthetic pathway, sulphate reduction, methanogenesis, bacterial contaminant breakdown, etc.), that select the light isotope

d

NB: the isotopic fractionation is

efficient when the light isotopes areabundant .

In closed (limited) systems or duringintense, rapid reactions the lightisotope pool is progressivelyconsumed, so that a competitionarises between light and heavyisotopes and the d of the reactionproduct tends to become similar tothe source d = «maturation»


→ Isotopic fractionation = the basis for the application of isotope studies in many fields including ecology…

…since each materials has an own isotopic signature in relation to the combination of natural isotopic proportion proportions and fractionation.

Once a reliable isotopic signature is known, it can be used to trace the origin of other substances, and temporal changes in physical, chemical or biological processes.

Isotopic fractionation in ecology

modified from Rutz et al., 2010

Terrestrial ecology:

C and N isotopic signaturesin food sources of natural crow populations

to track the ability of certain individuals to use tools to extract larvae from tree trunks

Isotopic signature vs bio-geochemical cycles - Carbon (13C/12C)

recommended reading: Peterson & Fry, 1987

a = ~ -22‰

→ are rather complex in aquatic ecosystems, but especially in lakes

Isotopic signatures → are strongly related to the bio-geochmical cycle of each element

The isotopic signature of Nitrogen (15N/14N)

• d15N in most parts of the biosphere ranges between -10‰and +10‰

• N supply rate to biological processes can be limiting → competition and scarce fractionation

• Complicated by nitrification, denitrification and fixation

• Increasing d15N in animals is due to preferential excretion of light N with urine

from Peterson & Fry, 1987

The isotopic signature of Sulphur (34S/32S)

from Peterson & Fry, 1987

ocean SO42- is now homogeneous, but over geological time ocean d34S changed

producing a variety of terrestrial S signatures

Lakes: H2S is produced by very selective reduction of SO42- under anoxic conditions

→ informa�on on redox conditions and indirectly on lake trophic conditions


Natural dynamics vs human-related impacts

Catchment processes

Pollution

Climate

Ecological processes

Nutrients

Tolottti et al., 2018 in rev.

Lake environmental and ecological condition and development are the result of interacting natural and human- related catchment and lake dynamics.


The study of lake sediments can help closing these gaps

Temperate akes: multiple human impacts vs. limnological gaps

Multiple stressors

• Nutrient enrichment• Climate variability• Pollutants and contaminants• Hydroelectric exploitation

complex lake responses

• additive/synergic interactions

• organism responses differin timing and intensity

+Knowledge gaps

• regular limnological surveys in the last few decades → short �me perspec�ve

• short regular meteorological records

• effects of new contaminants, or hydroelectric exploitation scarcely investigated

hamper

the interpretation of past and

current lake responses to

environmental drivers

capability of forecasting future

trends


Sediments as a lake historical archive

Investigating the lake sediments allows getting access to the lake historical archive

Remains and signs of physikal, chemical and biological events occurring in lakes are stratified and preserved in the deep sediments.


Paleolimnology objective

= known (measured, reconstructed, modelled)

= unknown

PresentPast Future

Limnologicalsurveys and monitoring

Sediment investigations

Prediction

General objective = to expand backwards (inferring) the knowledge on long term evolution of

lakes in relation to local and global forcing, and to use this information to

forecast future lake development.

The farther backward you can look, the further forward you’re likely to see.

Winston Churchill (1874-1965)

Applications of sediment studies

Reconstruction of lake responses to past:

nutrient enrichment (eutrophication)

pollutants (e.g. acid rains, heavy metals, POPs)

long-term scale (Holocene) climate variability

climate- and human related catchment processes (e.g. earthquackes, floods, erosion, deforestation, agriculture)

ecological processes and intereactions

Sediments of high altitude lakes are optimal for investigatinglong-term effects of climate change on lacustrine ecosystems:

a) scarce direct human impact (but... diffuse atmosphericcontamination!)

b) simpler food webs, processes, dynamics

L. Marmotte, Cevedale (I)


Lake reference conditions

WFD EU/60 2000: lake ecological reference conditions (before major human impact) have to be defined in order to set realistic quality targets (conservation, restoration)

Though past-oriented, paleoecology strongly depends on theknowledge of present relationships between organisms andenvironment, which is necessary to infer past environmentalconditions and to validate reconstructions.

• to disentangle lake responses to different impacts

• to assess lake vulnerability in relation to future nutrient inputs (scenarios)

• to contribute to the development of mitigation and adaptation strategies aimed at maintain ecological functionality and human use (research vs management)

Complementarity of paleo- and neo-limnology

Re

fere

nce

co

nd

itin

os

Lake Garda

from Milan et al., 2015


As a consequence of the climate change, the reference conditions may:

1. not been reached any more by restored lakes

2. change even in pristine lakes

The understanding of effects of past and present climate variability on lake ecological processes is of key importance for management purposes

badpoormediumgoodhigh

Qualitytarget

1 = recovering process without climate change2-4 = recovering scenarios under different climate changes3 = base line (reference conditions)

Bennion et al., 2011

Climate change as a new player

Paleolimnology: a multidisciplinary approach (I)

Biological proxies

Organism remains:

- vegetals: pollen, diatoms, resting cells (Chrysophyceae and Cyanobacteria)

- animals: Cladocera, Ostracoda, insects, molluscs, etc...

Biochemical remains:

- algal and bacterial pigments: primary productivity and diversity

- lipids : changes biodiversity and productivity

- DNA: biodiversity changes (diversity, microevolution), physiology

Proxy = synthetic indicators of limnological conditions and processes

Lithological proxies

visual aspect, varvae, granulometry, wet density: hydrology

Water and organic content, C (TOC + CO3), S, N, P: chemical and trophic evolution

Geochemical proxies

SCPs (Spheroidal Carbonaceous Particles): atmospheric contamination by fossil fuels

Heavy metals (Ni, Cr, Zn etc.), POPs (Persistent Organic Pollutants as PCBs, PAHs):

industrial and agricultural contamination

Radioactive and stable isotopes: chronology, acidification, eutrophication, food webs,

human pollution, climate change

DDT = dichloro-diphenil-chloroetahne Perylene (PAH)

Paleolimnology: a multidisciplinary approach (II)


Stable isotopes used in paleolimnology

Radioactive: 210Pb, 137Cs, 239Pu, 3H, 14C → core chonology

Geochemical: e.g. d87Sr, d208Pb and d207Pb, d202Hgto track evolution of hydrology, human-driven pollution (cf. Revel-Rolland et al., 2005,Yin et al., 2014 for a review on Hg)

e.g.: d207Pb = ~1.1 if soil-derived (natural), 1.5 if atmospherically derived (pollution)

Biologically active = H, O, C, N, S as components of biological matrices, they can be used to reconstructecological processes, response to climate change, changes in the lake food

web as related to webs as driven by environmental changes or human perturbations

Biologically active

in organic matter of bulk sediment

the fraction of organic matter that escaped degradation during and after sedimentation

C +N

in biological remains preserved in sediment

different isotopes according to matricese.g.: C and O in CO3

2-, Si and O in biogenic SiO2, etc...


Radiometric datingBased on g-emission of radioactive isotopes (no d!) in bulk sediment

Recent sediments (up to ~150 year old)

→ 210Pb: natural, originates from the 238U decay series through 222Rn and 226Ra (half life = 22.3 y)

Total Pb activity (measured as 210Pb) = supported + unsupported

supported = derived from in situ decay of the parent readionucluide 226Ra (measured as 226Ra)

unsupported = from 210Pb atmospheric deposition, calculated subtracting supported from total cf. Appleby, 2001 for a review and assumptions

Tolotti et al., 2018 in rev.

1963

1963

→ 137Cs: artificial from atomic industry, atmospheric fallout and deposition. Used for validating 210Pb chronology: peaks in 1963 (ban of atomic tests) and 1986 (Chernobil accident).

Pb dating horizon


Radiometric dating

Old sediments (> ~150 year, up to 40,000 y):

→ 14C: forms in the atmosphere by cosmic ray bombardment of 14N unstable, decays back to stable form with an estimated half life of 5730 ±40 yearsconventional half life = Libby half life = 5568 years

- taken up by plants as 14CO2 till equilibrium and transported through the food web- after organism death, replenishment stops and decay begins (“C clocks starts”)

• problem in lake sediments = contamination

a) by old C, e.g., organic matter stored in glaciers (high altitude lakes), or carbonate weathering in in calcareous catchments (“hard water effect”) can enter the lake food web

b) by young C, e.g through mosses roots, fungi, sediment perturbations

bulk sediment

terrestrial plant

from Björck et al., 1998

dating of less affected material (vegetal remains)and avoiding dating of bulk

cf. Björck & Wohlfram, 2001 for a method review


usually combined with:

- elemental analysis → total amount of organic C (TOC) and N

- indicators of lake productivity (OM, subfossil pigments, diatoms, Cladocera)

- contemporary data (temperature, precipitation, land use, pollution sources)

Provide information on long term evolution of:

- lake productivity (indirectly trophic status)

- climate variability

- atmospheric deposition of pollutants Nr (reactive N), C, S in less human impacted lakes

Stable isotopes in organic matter of bulk sediment

1) Dehydration of fresh sediment (freeze drying to avoid lost of volatile organic components)

2) Stable isotope ratio analysis (SIRA) of a few mg DW

supporting information

Bulk isotopes = the result of many simultaneous processes = tricky interpretation

Sediment organic matter and TOC (Total Organic Carbon)

Primary source of sediment organic matter = photosynthetic organisms

a) vascular in and around the lakes

b) non vascular (phytoplankton) with no or little cellulose

organic matter = lost on ignition (LOI) as eliminated by heating at 550°C in a furnace

TOC the fraction of organic carbon within tin the bulk organic matter

FWBulk fresh sediment

100%

CO3 = Ash –residue

Dry mass 10% FW

Water content<50-90%

FW

110°C Dry mass DW

organicmatter = LOI

Ash 90% DW

950°Cresidue

550°C

Ash 90% DW

LOI = DW - Ash

TOC = ~LOI/2

residue CO3


Accumulation affected by different factors:

e.g.: lake depth, sediment focussing on deepest lake sediments, sediment grain size and clay, oxygenation

1) Mass accumulation rate (MAR, related to sedimentation rate)

2) Relative changes (within a profile) more relevant than absolute values

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4

8

12

16

20

24

28

32

36

40

44

48

52

56

60

64

68

72

76

80

84

mg L-1

0

4

8

12

16

20

24

28

32

36

40

44

48

52

56

60

64

68

72

76

80

84

Depth

(cm

)

0.4

Sed

rate

1.6

WD

100

H2O

20

Organ

ic

300

CD

IsoB

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Age (

y)

1971

2011

2005

1981

1965

1988

1998

1935

1861

1783

1689

PI1

PI2

PI4

PI3

%fwg cm-3

g cm-2

y-1 %dw U gLOI

-1

Lake Ledro

Sediment organic matter

Both organic matter (LOI) and CO32- increase with enhanced lake productivity

Tolotti et al., 2016; original data

1963

Age (

AD

)

2015

1983

1922

2005

Depth

(cm

)

1851

Bq Kg-1 Bq Kg-1 g cm-2 y-1 % g FW-1 g cm-3 mg L-1

0

3

6

9 12

15

18

21

24

27

30

33 36

39

42

45

48

51

54 57

60

63

66

69

72

75

78 81

84

87

90

93

96

3500

137

Cs

10 100 1000

Tot 2

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Supp

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Pb

0.2

Sed ra

te

20 40

DW

1.4

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12 24

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LOI

0

3

6

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21

24

27

30

33 36

39

42

45

48

51

54 57

60

63

66

69

72

75

78 81

84

87

90

93

96

Lake Lugano

1960s


TOC/N (weight or atomic ratio) combined with d13C → provides origin of organic matter

from Meyers & Lallier-Vergès, 1999

TOC/N and d13C of bulk sediment organic matter

Major changes during sedimentation and early diagenesis:

- vascular plants: C/N decreases as labile lipids and sugars are degraded- phytoplankton: C/N increases as N-rich proteins are first degraded by bacteria - productive lakes: C/N increases with depth- oligotrophic lakes: C/N decreases with depth

However, C isotopic signature is maintained → Difference between vascular plants and phytoplankton persists in old sediments

Inorganic C source (DIC): CO2 d13C = -7‰

HCO3- d13C= 1-2 ‰

C3 plant more effective in selecting 12C (use more easily 12C)

→ organic ma�er is more nega�ve (lighter)

→ remaining DIC pool tends to becomeheavier (more positive)

TOC and C/N: example

2010

2008

2005

2002

1998

1993

1987

1980

1974

1968

1963

1956

1947

1939

1894

1873

1915

1928

Ag

e (

yea

r)

% dwg cm-3g cm-2 y-1 nmol LOI-1 ww% dwN 103 g dw-1

N 103 cm-2 y-1

0 2 4 6 8

10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68

De

pth

(cm

)

0.3 0.6

Sed

imen

tatio

n ra

te

1 2 3

SC

Ps

1.5 2.0

Wet

den

sity

70 100

H2O

15 30

Org

anic

mat

ter

5 10

Tot

al-C

TO

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0.3 0.6

TN

45 90

TO

C :

TN

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: 410

25 50

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0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68

Lake Neusiedler = large (~500 km2) , shallow (zmax = 1.8 m) lake in Eastern Austria (Pannonia)Evolution of reed belt during the last century, max cover in the 1960spresent = ~320 Km2 open water 180 Km2 reed belt

from Tolotti et al., in prep.

d13C in organic matter of bulk sediment

C3 plants (phytoplanklton): d13C < -25 ‰

when availability of dissolved CO2 is scarce (increased primary productivity, alkaline pH, high altitude,) the more abundant DIC resource becomes HCO3

- (d13C ~ 1-2 ‰)

further produc�vity → compe��on for C → consumption of 13C (enriched residual DIC) → smaller fractionation and increase in d13C in the organic matter

Multi-factorial control of d13C multiproxy approach is necessary when approaching

isotope studies in bulk sediments

increase in produc�vity → more posi�ve d13C, up to - 9‰

changes in pH, temperature, nutrient limitation, and growth rate affect the uptake of 13C

e.g.: inputs of HCO3 from the watershed, increase in benthic production, macrophytes, lead to more positive d13C of lake organic matter

Diagenetic effect is minimal on d13C (though up to 20% on composition of organic content)

d13C in bulk sediment – example I

from Meyer & Lallier-Vergès, 1999 recommended review incl. examples

Lake Baikal

SiO2 n(H2O)

a) lake productivity increased after glacier retrat and climate recovery

a)

b)

b) contribution from land vascular plants dominant under glacial climate, while algal become dominant in the post glacial period

c)

c) C4 plants were abundant during the glacial conditions (dry and cold weather tundra grasses)confirmed by palinology

d) Holocene phytoplankton has the typical C signature od C3 plants

d)

!! d13C (plants) = d13C (phyto) !!

d15N in organic matter of bulk sediment

biogeochemical N-cycle is more complex than C-cycle

→ interpretation more difficult

→ d15N not as widely used as d13C in bulk sediment

useful to complement d13C information on origin of organic matter based on difference between the isotopic conntent of the inorganic N pool and plants

increase in algal produc�vity → increase in d15N (competition for N and low fractionation)

high variability introduced by N-recycling = factors other than primary productivity.

• denitrification in anoxic waters and sediments enriches the residual DIN with 15N and d15N of organic matter becomes more positive

• N2 fixing by cyanobacteria is highly selective for 14N and decreases d15N values

• inputs of heterotrophic organic matter (d15N > ~10 ‰) increases the d15N of sediment organic matter and can mimic increasing primary productivity

d15N dissolved NO32- = +7 to 10‰

atmospheric N2 = 0‰

d15N plankton around +8‰

N fixing land plants +1‰.

Crater Lake Bosumtwi, Ghana (closed system)

d13C and d15N in bulk sediment - example


C4

land surce

in-lake

plants

mix C3

Increased d15N during the dry glacial age climate:

a) evaporation of volatile light NH4 (d15N = -15 to +3‰) → organic ma�er produced from a

heavier DIC reservoir

b) depletion of the N reservoir due to diminished input of soil nutrients during the dry galcial period (N limitation) and scarcer fractionation. Mimics increased primary productivity.


Crater Lake Bosumtwi, Ghana (closed system)


C4

land surce

in-lake

plants

mix C3 N2 fix

Decrease in d15N during the post glacial period with very stable thermal stratification:- dominance of N2 - fixing cyanobacteria → strong isotopic frac�ona�on

Present day situation = intermediate conditions- wind mixing maintains a supply of DIC wich sustain agal growth

d13C and d15N in bulk sediment - example

d15N in bulk sediment of remote lakes

High al�tude and la�tude lakes → simple, oligotrophic systems with scarce direct human impact

Lakes of the N-hemisphere → highly coherent decrease in d15N which parallels the record of atmospheric deposition (C + N) preserved in the Greenland Ice Sheet

Nr = atmospheric reactive N (NH3, NO, NO2) transported as NH4+, HNO3, NO3

- and with depleted d15Noriginating from fossil fuel since ca. 1850 and from N-fixation since the 1950s.

Increase in C and N anthropogenic emission during the last 100 years = threshold for defining the Anthropocene (human alteration of biogeochemical cycles)

from Holtgrieve et al., 2011

Fossil fuel N-fixation


Isotopic signature in bio-geochemical remains

persistent component of the biota (e.g. shells, frustules, head capsules) +

bio-geochemical components, such as CaCO3, lipids, chlorine (pigments) are produced over a short time (life time of aquatic organisms = weeks, months)

→ their isotopic composi�on can store more reliable information on specific past lakeenvironmental conditions/processes

d13C and d15N in bulk sediments → result of a mixture of different processes → complex interpretation

Still…interpretation remains highly site specific!

→ used to track past changes in the lake food webs (in combination with classical paleoecological techniques)


Long term changes in the lake C-cycle - d13C

Lake Annecy (F): eutrophication + re-oligotrophication accompanied by changes in the food web

modified from Frossard et al., 2013

• Cores → Deep (65 m) + sublittoral (30 m)

• benthic and pelagic consumers

• Bayesian change point analysis (bcp R package)

collector-filterer predator filter feeders

Pre 1930:- highest d13C in all species and cores- minimum differences between cores

- decrease in d13C in all taxa duringeutrophication

Post 1950:- heterogeneous stage- littoral stabilization, deep further decrease

coll.-gatherer

Chironomid head capsules + Cladocera remains

Handpicked + chitin d13C analyses

1940

- Cladocera: only 2 stages (1940)


Long term changes in a C-cycle - d13C

- d13C = ~ 30‰ agree with atmospheric DIC source (CO2) for both pelagic and benthonic (deep and littoral) organisms

Pre-eutrophication

modified from Frosard et al., 2013

Lake Annecy (F): eutrophication + re-oligotrophication accompanied by changes in species composition (hysteresis)

- Expected = increasing d13C due to smaller fractionation in phytoplankton

- Decreasing d13C is unexpected

- Increased primary production enhanced heterotrophic bacterial respiration during stratification

Eutrophication Re-oligotrophication

- d13C remains low = no recovery

- further decrease in the deep core: C source = methane derived from methanogenesis in the anoxic hypolimnion


Stable isotopes in biogenic silica (diatom frustulum) - d18O

Diatom frustulum = 2 layers:

inner = tetrahedrally bonded Si-O-Si, O incorporated during silicification

outer = hydrous Si-OH-Si, O is exchangeable with water andmust be removed before analysis

d18O(diatoms) is a function of: 1) water temperature (from bulk samples: d18O ~ -0.2‰ °C-1)Crespin et al., 2010 recommended

2) isotopic composition of lake water during silicification= precipitation, runoff, groundwater and evaporation, mixing regime, which in turn depends on temperature

NB: precipitation has negative d18O, strong evaporation enriches water with d18O

residence time will also influence the magnitude of enrichment

Reliable interpretation of diatom d18O needs strong support from environmental data (climate variability, weather conditions, hydrological balance)

highly site specific

Source = orthosilicic acid H4SiO4 SiO2 + H2O → H2SiO3 +H2O → H4SiO4 (quartz hydration)


Three stable Si isotopes, 28Si, 29Si and 30Si (d30Si = 1.96 - d29Si)

Both geochemical precipitation in minerals and bio-minarelization in soil, rivers and lakes →

• preferential incorporation of light 28Si (fractionation)• d30Si enrichment first in soil moisture and then in lake water

d30Si enrichment factor = -1.1 ‰ to -2.0 ‰

→ informa�on on: - Si availability and utilization rate in the lake euphotic zone (Si limitation)

- relation between terrestrial and water Si cycling (as diatoms are a Si-sink)

Stable isotopes in biogenic silica (diatom frustulum) - d30Si

Especially studied in oceans as global Si cycle is strongly controlled by diatoms

(>40% of the total primary production)

Università Milano Bicocca, May 21th 2018modified from Street-Perrot et al., 2008

L. Rutundu, crater lake on Mount Kenya

Ho

loce

ne

Gla

cial

d30Si (diatoms) is affected by: - lake catchment geology and land cover (weathering and runoff) - water residence time- nutrient fertilization (diatom growth becomes limited by Si)- nature and seasonality of diatom blooms

Background datasite-specific results

→ scarce studies!

Stable isotopes in biogenic silica (diatom frustulum) - d30Si

C limitation

C3

N2 fix

1) dry climate (low precipitation and higher evaporation) 18O enrichment

1)

b) Humid late glacial conditions: enhanced precipitation (low evaporation)

2)

3) Glacial stage:

high d30Si → low runoff (cold) and strong Si lmitation

Low d30Si → direct Si input from volcanic springs (not yet enriched by mineral precipitation)

Diatoms decreased in Holocene, less Si runoff from the vegetation-covered catchment

3)


d13C and d18O in carbonates

Ocean → routine analisys for reconstruction of past climate and changes in sea level

Lakes → less common for mixed origin (inorganic + biogenic) and complex interactions

- carbonate fractions need to be first characterized and quantified (X-ray diffraction)- supporting evidence from other sediment proxies- knowledge of processes operating in the modern lake (physical, chemical properties, climate)

Inorganic CO32- →

• endogenic (precipitates in the water mainly due to photosynthetic uptake in the epilimnion. Is the sole which is in thermodynamic equilibrium with lake-water)

• detrital (catchment origin)

• diagenesis within the sedimentDifficult separation and needs support from analyses of carbonates in the catchment

Biogenic CO32- → molluscs, Ostracoda, charaphytes (green algae)

• They precipitate CO32- with energy costs and with some isotopic fractionation

(vital effect = deviation from the expected thermodynamic equilibrium)

• Mollusc shells provide an average d13C for their life span

• Ostracoda moult 8 time before getting adult (physiological response, seasonality); vital effect on d18O

• Characeae and other green algae Individual isolation (at species level!), knowledge of life cycle


Stable isotopes in molecular biomarkers

d13C in lipids → n-alkane, and n-alkanols (C-OH), n-alkanolic acids (COOH)C27, C35 n-alkane in vascular plants (coa�ng waxy molecules) → C3 vs C4, climate C27, C35 n-alkane in aquatic macrophytes (water level, salinity)C17 -C21 n-alkane in algae and photosynthetic bacteria (productivity)

Relies on - extraction (GC/MS) and quantification of organic molecules preserved in the sediment- possibility to relate them to a precursor organism

dD in lipids → proxy of past climate and hydrological variability:

climate change affects in different ways the the dD ofmeteoric and lake water → registered in dD plant and algal lipids. Fractionation by algae: dD = -57 tp -220‰

Leng & Henderson, 2013 for an overview

Combined with analysis of the abundance of FAs (fatty acid) including sterolsBechtel & Schubert, 2009

Lake Brienz (CH)


d13N in amino-acids (cores + sediment traps) from marine ecology, first application to lakes = Carstens et al., 2013

- comprise a large fraction of the organic N in plankton and sediments (marine ecology!)- fractionation during metabolic processes and trophic transfer

not enriched → informa�on on N sourcesenriched → informa�on on metabolic processes and transfer in the food web

biochemical and metabolic knowledgecoupling with source analysis and sediment traps for diagenetic processes

Isotopes in cellulose → the most abundant bio-molecule (vascular plant and some green algae)d13C and d13N, and C-bound dD are “locked” in dead cellulose paleo-hydrology

- fractionation between water and cellulose is known and ratherindependent from temperature and plant type (acell-water ~1.025, lab exp.)

- multi-step complex preparation: acidification, sieving, solvent extraction…

- condition: no incorporation of terrestrial cellulose (catchment)

Applica�ons → similar and complementary to isotopes in diatoms- d13C for reconstruction of past C balance- d18O, dD for reconstruction of precipitation (indirectly temperature, and climate)

Wolfe et al., 2001 for introduction

Stable isotopes in molecular biomarkers


• Stable isotope analysis of molecular biomarker → rapidly evolving

- complex, multi-step isolation and purification (specialized labs, expensive) - strong support necessary from contemporary data, experimental approach for processes- complex interpretation (biochemistry, metabolic processes)

Future perspective

• More reliable and rapid (automatized) measurement instruments (smaller amounts, improved accuracy)e.g.: first precise measurements of d202Hg in natural samples in 2000!

• Subfossil remains → improved techniques for sample preparation and matrices purification (e.g. for biogenic Si and O from diatom frustules, cf. Leng & Henderson, 2013)

• Multiple isotope ratios in the same aliquot subfossil remains (d13C , d15N, d18O, dD). E.g. d18O, dD in chironomid head capsules similarly as in diatom frustules (reconstructionof past air temperature and source of precipitation, food sources)

Stable isotope analyses are becoming more fully incorporated in paleolimnological studies

Still no routine but very promising for the future!

To take home

stable isotope analysis developed and is still better established in marine ecology application is vast and rapidly growing paleolimnology, already routine for bulk sediments

Nothing is absolute when studying isotopes:

- in bulk sediments → many simultaneous processes over a wide �me span

- in subfossil remains → temporal window is smaller, processes can be focussed

- in biomarkers → complex prepara�on difficult interpreta�on

risk = the effort may not be compensated by clear results (all smoke and mirrors)

objectives must be very clear before planning and undertaking stable isotope analyses

careful interpretation and generalization of results :

→ multidisciplinarity is necessary for interpretation and validation

→ knowledge of the present environmental and ecological context

→ highly site specific (different evolution, processes, impacts)

powerful tool for reconstructing and interpreting past processes behind changes

can help overcoming a shortcoming of paleolimnology


Thanks for the kind attention!

Acknowledgement

CEU Project EuLakes [Ref. Nr. 2CE243P3]

Interreg IV Italia-Austria PERMAQUA [CIG: 40468649CF]

Adriano Boscaini, Manuela Milan, Margherita Obrelli (FEM) for their help in the field and the lab


d13C TOC and C/N in bulk sediments of remote lakes

Großer MalerseeRaintal, 2501 m

Wilder PluderseeUltental, 2493 mALTO ADIGE

Nordtyrol (A)

PLU

MAL

Deglaciated catchment

mean EC: 182 mS cm-1

pH: 7.2-7.6Oligotroophic: mean TP: 5 mg L-1

Bolzano

Interreg IV Italy-Austria (2011-2015)

www.permaqua.eu

Coring: October 2012

23 cm long

Dating: 210Pb, 137Cs, 14C

Lithological, geochemical, biological proxies

C + N isotopes on bulk organic matter


Ag

e (

ca

l. Y

ears

BP

)

83 ± 7

0

9 ± 2

58± 4

128 ± 25

39 ± 2

21 ± 2

1992 ± 54

2895 ± 8

3367 ± 75

De

pth

(cm

)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

1 2

WD

ens.

10 20

Org

. (LO

I)

7.0 7.5

DI-p

H

6 8 10

DI-T

P

% DW mg L-1g cm-3

TOC/N and d13C in bulk sediments of remote lakes

After ~ 1850 AD(# 1-14 incl . 29 30 31)

Before 1850 AD

10 35 70

C T

ot

TO

C

2 4

CO

3

4 8 12

TN

10 15 20

TO

C/T

N

-20 -15

d13C

(or

g)

2 4

d15N

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

g Kg-1 %g Kg-1 d13C d15N

NB: Slight reduction in N atmospheric depositionsince ~2000

d15N in bulk sediment of remote Alpine lakes

Age (

cal.

Years

BP

)

83 ± 7

0

9 ± 2

58± 4

128 ± 25

39 ± 2

21 ± 2

1992 ± 54

2895 ± 8

3367 ± 75

Depth

(cm

)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

-15 2 4

d15N

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

d15N

Maler

Coherence!Increase in d15N during the last 20 years = increase in algal productivity?

Age (

cal.

Years

BP

)0

6 ± 2

30 ± 2

12 ± 2

20 ± 2

42 ± 2

54 ± 3

58 ± 3

63 ± 4

73 ± 5

84 ± 6

103 ± 7

128 ± 9

155 ±16

(335 ± 16)

Depth

(cm

)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

-15 -10

d13C

(or

g)

0 2 4

d15N

d13C d15N

Pluder


5 10

Navi

cula

ten

ello

ides

5 10

Ach

nanth

idiu

m m

inutis

sim

um

5 10

Sta

uro

neis

sm

ithii

3 6

Navi

cula

cript

oce

pha

la

7 14

Am

phor

a in

arie

nsi

s

3 6

Ency

onem

a p

erp

usi

llum

10 20

Am

phor

a c

opu

lata

20 40

Ency

onem

a s

ilesi

acu

m

4 8

Navi

cula

det

enta

20 40

Cavi

nula

lapid

osa

/vario

striata

5 10E

ncy

onem

a m

inut

um

3 6

Nitz

schia

alp

inobaci

llum

25 50

Sta

uro

sira

mic

rost

riata

5 10

Sta

uro

sira

vent

er

5 10

Eolim

na readeria

15 30

Sta

uro

sira

par

asi

toid

es

20 40

Sta

uro

sira

pse

udoco

nst

ruens

25 50

Sta

uro

sira

m

uta

bili

s

4 8

Eolim

na s

p

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

%

D7

D6

D5

D4

D3

D2

D1

Age (

cal.

Years

BP

)

83 ± 7

0

9 ± 2

58± 4

128 ± 25

39 ± 2

21 ± 2

1992 ± 54

2895 ± 8

3367 ± 75

Depth

(cm

)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

-15 2 4

d15N

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

d15N

Maler

Recent species (Staurosira) requires higher N concentrations and are meso-eutraphentic




0.000

5.000

10.000

15.000

20.000

25.000

1 2 3 4 5 6 7 8 9 10111213 Depth (cm)

DAR 10^6 (Diatom Accumultion Rate) N cm-2 y-1

0

5000

10000

15000

20000

25000

1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728293031323334353637383940414243444546

Depth (cm)

Total diatoms per g DW 10^6

Age (

cal.

Years

BP

)

83 ± 7

0

9 ± 2

58± 4

128 ± 25

39 ± 2

21 ± 2

1992 ± 54

2895 ± 8

3367 ± 75

Depth

(cm

)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

-15 2 4

d15N

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

d15N

Maler

stable isotopes in profundallake sediments applications

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