Accumula'on-rate changes through the Holocene from ice-core records and radar data. Michelle Koutnik Earth and Space Sciences

Transcription

1 Accumula'on-rate changes through the Holocene from ice-core records and radar data Michelle Koutnik Earth and Space Sciences

2 Outline Basics deriving accumula9on histories from ice-core records Ice and accumula9on histories from Central WAIS Extending ice-core accumula9on records spa9ally with radar Central West Antarc9ca (shallow layers) Taylor Dome (deep layers) Context for recent climate change Antarc9c peninsula Holocene temperature history Last 2, years of WAIS ice and climate change

3 Why this ma9ers Ice-core records have high temporal resolu9on; compare independent records to each other Specifics of past climate are a necessary boundary condi9on to run ice-evolu9on models for ice-sheet reconstruc9ons Radar-detected internal layers are an increasingly abundant data set that can add spa9al informa9on Holocene climate change gives an interes9ng and necessary context to modern records

4 Takeaway points Deriving climate histories from ice-core records requires knowledge/assump9ons about the ice-sheet history

5 Takeaway points Deriving climate histories from ice-core records requires knowledge/assump9ons about the ice-sheet history Ice cores are tremendous archives but using them with radar data brings dis9nct value (for past and recent change)

6 Takeaway points Deriving climate histories from ice-core records requires knowledge/assump9ons about the ice-sheet history Ice cores are tremendous archives but using them with radar data brings dis9nct value (for past and recent change) Gain insight into what the con9nent experienced, then why

7 Takeaway points Deriving climate histories from ice-core records requires knowledge/assump9ons about the ice-sheet history Ice cores are tremendous archives but using them with radar data brings dis9nct value (for past and recent change) Gain insight into what the con9nent experienced, then why Ice-sheet model reconstruc9ons constrained by ice cores

8 Takeaway points Deriving climate histories from ice-core records requires knowledge/assump9ons about the ice-sheet history Ice cores are tremendous archives but using them with radar data brings dis9nct value (for past and recent change) Gain insight into what the con9nent experienced, then why Ice-sheet model reconstruc9ons constrained by ice cores Temperature histories provided by ice-core data put modern change in context with the past

9 Deep ice-core sites + South Pole + Roosevelt Island + James Ross Island Cuffey and Paterson (21)

10 Where to drill an ice core theore&cally Assume uniform strain rate and no mel9ng Accumula9on constant In reality, strain rate varies with depth Cuffey and Paterson (21)

11 Cuffey and Paterson (21)

12 Basics of deriving a climate record: Age scale Annual layer coun9ng (chemistry, dust, electrical conduc9vity) Volcanic, cosmogenic, and other marker horizons Markers related to globally mixed gases Oxygen and hydrogen isotopes of ice Physical characteris9cs (e.g. bubbles) Ice-flow model with depth-varying strain rate

13 Holocene ice-core records across Antarc'ca Masson et al. (2)

14 Holocene ice-core records across Antarc'ca Ice-sheet eleva9on changes may be superposed on common clima9c fluctua9ons Masson et al. (2)

15 Basics of deriving a climate record: Accumula9on rate Need to apply ice-flow models in order to es9mate accumula9on history at a core site from measured layer thicknesses S(t) 1-D 2-D 3-D

16 Basics of deriving a climate record: Accumula9on rate Need to apply ice-flow models in order to es9mate accumula9on history at a core site from measured layer thicknesses S(t) 1-D 2-D 3-D

17 Basics of deriving a climate record: Accumula9on rate Separate the influence of ice flow vs. climate on ice-core record Some'mes this is easy but, some&mes this is hard

18 Interior of Antarctica Assume that we can describe ice flow in ice-sheet interior 35 Ice divide 3 25 Elevation (m) Distance (km)

19 Interior of Antarctica As the ice sheet evolves over thousands of years, accumula'on rate and flow rate change 35 Ice divide 3 25 Elevation (m) Distance (km)

20 Interior of Antarctica The rate, pa9ern, and 'me-varia'on of climate and ice dynamics affect par'cle paths 35 Elevation (m) Par&cle paths Internal layer (isochrone) Distance (km)

21 Interior of Antarctica Deeper layers from further back in 'me Elevation (m) Distance (km)

22 Interior of Antarctica Deeper layers from further back in 'me Elevation (m) Distance (km)

23 Interior of Antarctica Deeper layers from further back in 'me Elevation (m) Distance (km)

24 Interior of Antarctica Dynamic and clima'c histories recorded in the internal layers and in an ice core: 35 Elevation (m) Internal layers Ice core At the divide OR on the flank Distance (km)

25 Flowband (ice flow) model We can use the ice-core record and layers together with an ice-flow model to constrain ice-sheet history Inputs: Accumulation. rate, b(x,t) Surface Temperature, Ts(x,t) Outputs: q(x,t) L Ice thickness, h(x,t) Ice divide Width, W(x) q(x Flux R,t) changes due to dynamics z y x Heat flux, Qg(x). Melt rate, m(x,t) Bed, B(x)

26 Central West Antarctica Koutnik et al. (216)

27 Central West Antarctica: Data Time (μs) b) 1 Time (μs) WAIS Divide ice core (WDC) Distance (km) 4 c) Height above the bed (km) 7Mhz and 1Mhz radar data a) Traced internal layers 3.5 Depth-age data d) Distance (km) Age (kyr BP)

28 Central West Antarctica: Data a) Time (μs) Time (μs) WAIS Divide ice core (WDC) Distance (km) 4 Height above the bed (km) c) b) d) Distance (km) Age (kyr BP)

29 Central West Antarctica: Data What is the Holocene accumula'on history at the ice-core site? Ice divide is migra'ng today, did it migrate in the past? a) Time (μs) Time (μs) WAIS Divide ice core (WDC) Distance (km) 4 Height above the bed (km) c) b) d) Distance (km) Age (kyr BP)

30 Central West Antarctica Accumulation rate (m/yr) b) Relative elevation (km) kyr parkcle path a) Distance along flowband (km) Inferring ice and climate histories from the ice-core record must be done carefully 1. Ice in core originated upstream 2. Apply thinning func9on from a 1-D model (Buizert et al., 215) Then, advec9on correc9on to es9mate accumula9on history

31 Central West Antarctica: Simple estimate of accumulation Ice-core accumula'on rate: Accumula9on at loca9ons where ice in core originated upstream Accumulation rate a) (m yr -1 ice equivalent) Advection correction factor Ice-core accumulation rate Climate accumulation rate Annual data available we apply 1-yr filter Age (kyr BP) b)

32 Central West Antarctica: Simple estimate of accumulation Ice-core accumula'on rate: Accumula9on at loca9ons where ice in core originated upstream Climate accumula'on rate: Accumula9on at fixed loca9on (core site) Climate accumula'on rate = Ice-core accumula'on rate Advec'on correc'on factor Accumulation rate a) (m yr -1 ice equivalent) Advection correction factor Ice-core accumulation rate Climate accumulation rate Annual data available we apply 1-yr filter Age (kyr BP) b)

33 Central West Antarctica Inferring ice and climate histories from the ice-core record must be done carefully 1. Ice in core originated upstream 2. Apply thinning func9on from 1-D model and then simple advec9on correc9on 3. Use climate accumula9on history in 2.5-D model to generate layers 4. Do modeled layers match observed?

34 Central West Antarctica: Flowband model calculations Non dimensional depth a) Steady state Divide 9.2 kyr parkcle path WDC b) -1 m +5 m Divide WDC Divide WDC Distance along flowband (km) c) ~2 m Layer mismatch: (Modeled Observed) [m] Modeled layers too shallow Modeled layers too deep

35 Central West Antarctica: Flowband model calculations Non dimensional depth a) Steady state Using ice-core Using climate accumula'on rate accumula'on rate Divide 9.2 kyr parkcle path WDC b) -1 m +5 m Divide WDC Divide WDC Distance along flowband (km) c) ~2 m Layer mismatch: (Modeled Observed) [m] Modeled layers too shallow Modeled layers too deep

36 Central West Antarctica: Flowband model calculations Non dimensional depth a) Steady state Using ice-core Using climate accumula'on rate accumula'on rate Divide 9.2 kyr parkcle path WDC b) -1 m +5 m Divide WDC Divide WDC Distance along flowband (km) c) ~2 m Layer mismatch: (Modeled Observed) [m] Modeled layers too shallow Modeled layers too deep

37 Central West Antarctica: Flowband model calculations Non dimensional depth a) Steady state Using ice-core Using climate accumula'on rate accumula'on rate Divide 9.2 kyr parkcle path WDC b) Compare mismatch profiles at the WDC site -1 m +5 m Divide WDC Divide WDC Distance along flowband (km) c) ~2 m Layer mismatch: (Modeled Observed) [m] Modeled layers too shallow Modeled layers too deep

38 Central West Antarctica: Flowband model calculations Using climate accumula'on history gives a really good fit to internal layers ( within uncertainkes in layer depths from data colleckon, processing, travel-kme to depth, and layer picking: ± 8-32 m ) Non dimensional depth WDC site Steady state Using ice-core accumulation Using climate accumulation Layer mismatch: (Modeled Observed) [m]

39 Central West Antarctica: Model sensitivity Evaluate the goodness of fit at WDC to poorly known model parameters: Width func'on Non dimensional depth a) WDC (Ross Sea side) Layer mismatch: (Modeled Observed) [m] 3 Normalized width Basal melt (cm yr 1 ) Divide Position 1.5 b) Distance along flowband c) (km) d) Distance along flowband (km)

40 Central West Antarctica: Model sensitivity Evaluate the goodness of fit at WDC to poorly known model parameters: Holocene ice-divide posi'on Non dimensional depth a) 7 WDC (Ross Sea side) Layer mismatch: (Modeled Observed) [m] 5 4 Normalized width Basal melt (cm yr 1 ) Divide Position 1.5 b) c) d) Distance along flowband (km) Distance along flowband (km)

41 Central West Antarctica: Model sensitivity Evaluate the goodness of fit at WDC to poorly known model parameters: Basal melt rate Non dimensional depth a) WDC (Ross Sea side) Layer mismatch: (Modeled Observed) [m] Normalized width Basal melt (cm yr 1 ) Divide Position 1.5 b) d) c) Distance along flowband (km)

42 Central West Antarctica: Model sensitivity Different width func9ons and basal melt rates may also be consistent with layers different Holocene divide posi'on is not consistent Non dimensional depth a) 7 WDC (Ross Sea side) Layer mismatch: (Modeled Observed) [m] 4 Normalized width Basal melt (cm yr 1 ) Divide Position 1.5 b) d) c) Distance along flowband (km)

43 Central West Antarctica: Results 1. Accumula9on rate was ~2% lower than present at 9.2 kyr BP, and there was ~4% increase from kyr BP à big change 1.3 Normalized Accumulation Rate Dome C EDML Talos Dome Vostok WDC Scaled to mean of WDC for past 5 yrs Age (kyr) Talos Dome

44 Central West Antarctica: Results 2. Change in accumula9on leads to change in interior ice thickness unless compensated by a change in ice dynamics Accumulation rate scaling factor Surface elevation change (m) a) b) Time (kyr BP) ** Calculated eleva9on change depends on ini9al condi9on

45 Central West Antarctica: Results 3. The average Holocene divide posi9on likely remained within ~5 km of the modern posi9on Thinner The bedrock topography may have macroscopic control on ice thickness and divide posi9on Thicker 2 km

46 Central West Antarctica: Results Annual layer-counted 9mescale allowed for the first independent accumula9on reconstruc9on back to 31 ka 3 Extend prior to the Holocene Accumulation rate (cm/yr) Time (ka) Fudge et al. (216)

47 Central West Antarctica: Implications 1. To infer the most realis9c accumula9on history from the ice-core record, need to correct for advec9on of ice from upstream 2. Climate models extrapolate West Antarc9c climate from East Antarc9c ice-core records we can now do beler 3. Con9nent-scale model reconstruc9ons should use this climate forcing, and calculated Holocene changes in ice geometry should include a rela9vely stable divide near WDC

48 Central West Antarctica: Implications 1. To infer the most realis9c accumula9on history from the ice-core record, need to correct for advec9on of ice from upstream 2. Climate models extrapolate West Antarc9c climate from East Antarc9c ice-core records we can now do beler 3. Con9nent-scale model reconstruc9ons should use this climate forcing, and calculated Holocene changes in ice geometry should include a rela9vely stable divide near WDC What do con'nent-scale reconstruc'ons show?