1 DRAFT What have we learnt from the Paleo/Historical records. Kurt Lambeck Background One of the aims of workshop is to identify and quantify the causes contributing to the present observed sea-level change (rise and variability) such that better predictive models can be developed. Predictive in the current context is mainly for the future but more broadly it also includes the prediction of sea levels in the past for locations or epochs for which there is no observational data. We know that Earth has a memory of past events. Sea level change is not exempt from that. Sea level changes today, and will do so in the future, because of past events whether this be from the crustal response to past deglaciation, coastal subsidence from past sediment deposition, or from present tectonic upheavals whose underlying processes have their origins in the remote past. Thus to understand the present and future records we must understand the past record. We are currently in an interglacial, possibly towards the end of it. So that for present and future predictions our emphasis on the palaeo record is for interglacial periods, particularly the latter part of the Holocene during which the global ocean volume stabilised near its present value. But, while not the focus of the present workshop, we should also be aware of the Last Interglacial record. Is there evidence for sea level fluctuations towards the end of that period that herald the onset of the next glacial phase? If we can establish a global record of change for, say, the past 7000 years, what can we do with this? It provides a reference surface for quantifying rates of vertical tectonic movements, their episodicity as well as their longer-term average rates. It provides a means of evaluating climate-forced signals: are there global signals that can be attributed to regional climate indicators such as the little ice age or the middleages climate optimum. Are there regional changes in sea level associated with longterm changes in atmospheric circulation such as in the North Atlantic Oscillation? Can it provide evidence for global changes in ocean volume that are indicative of global changes in ocean-thermal structure or of changes in global ice volumes? In preparing this WCRP Workshop program some key questions identified were: 1. What changes in ocean volume (or eustatic sea level) have occurred over the past few thousand years since the termination of deglaciation of the major ice sheets? 2. If changes in ocean volume have occurred what has been its source (or sources)? Has thermal expansion played a role during climate optima in this interval or has it been the result of further but slow deglaciation of Greenland, Antarctica or mountain glaciers.
2 3. Has the eustatic sea level change been uniform or variable during this interval. 4. What are potential sources for new information on high-resolution and high-accuracy palaeo sea level change. 5. What other high-frequency contributions to relative sea level occur that may mask the eustatic signal? There may be others that should have been asked and we can add these as we go along. Some general issues. Sea level as recorded in the geological environment and historical documents is a relative measurement: of the shifting relationship between the ocean surface and land. For the man in the street this is all that is of interest: Will I get wet feet when I get out of bed in the morning. To understand what is going on requires knowledge of the movements of the two components. Observationally this can be done only by comparing altimeter and tide gauge results. Otherwise it has to be done through modelling the geophysical (including meteorological and oceanographic) processes involved. Averaged over long periods sea level is an equipotential surface whose shape is determined by the mass distribution within the earth, the oceans and any surface loads such as ice sheets. What is long within a centennial time scale needs discussion because superimposed upon this will be long and short-term departures from the equipotential surface caused by climate-forced redistribution of mass within the oceans (winds, thermal expansion). The observation of sea level change therefore contains information on (i) land movements, (ii) mass redistribution or geoid changes and (iii) changes in ocean volume or in distribution of water within the ocean basins. At this workshop we are primarily interested in (iii) but we cannot avoid (i) and (ii). Sea level change is globally variable and one ignores this variability at ones peril. When we characterise sea-level change, whether today, tomorrow or yesterday, we have to specify the spatial variability and a single number estimate will only be a crude approximation at best or wholly misleading at worst, depending on how it has been derived. A useful estimate for past global sea level is the area-average value over the ocean resulting from, for example, the melting of large ice sheets. In this case the icevolume-equivalent sea level is defined as!" esl (t) = # $ i 1 % $ t o A o (t) dv i dt dt
3 where V i is the ice volume at time t, A o (t) is the ocean surface area at time t, and ρ i, ρ ο are the average densities of ice and ocean water. In the absence of any other factors that lead to changes in ocean volume the ice volume equivalent sea level is equal to eustatic sea level. Sea level at any other locality and time is then defined as ζ rsl (ϕ,t) = Δζ esl (t) + Δζ I (ϕ,t) + Δζ Τ (ϕ,t) where ζ rsl (ϕ,t) represents the change at location ϕ of the sea surface relative to land at time t compared to its present position at time t P. The second term, Δζ I (ϕ,t), is the (glacio-hydro) isostatic contribution, and the last term is a tectonic contribution for tectonically active areas. Both of these terms are functions of position ϕ and of time t. In the absence of a complete observational record to relate information from one location to the next requires geophysical interpolation models. This requires knowledge of the ice history during the glacial cycle, of the earth response to loading on time scales of hundreds to thousands of years, and the means of evaluating the tectonic contributions. In a first approximation spatially variable Δζ I (ϕ,t) are largely a function of the distance from the ice sheets (the glacio-isostatic part) but short-wavelengths, of the order of lithospheric effective elastic thickness, also occur far from the ice sheets due to the changes in water loading (the hydro-isostatic part). But both parts are predictable if the growth and decay history of the continental ice is known and the pattern of the changes is well established. In contrast, the changes wrought by tectonic processes, of shorter wavelength and more episodic than the glacially driven change, tend to be less predictable and if the aim, as it is here, is to understand the climate-driven processes, then we should stay away from tectonically active areas or, alternatively, design the experiment such that we can remove the tectonic component. Writing ζ I (ϕ,t) = Δζ I-g (ϕ,t) + Δζ I-h (ϕ,t) where Δζ I-g (ϕ,t) is the glacio-isostatic part and Δζ I-h (ϕ,t) the hydro-isostatic part, the sea levels at two locations ϕ 1, ϕ 2 are ζ (1) (ϕ 1,t) = Δζ 0 esl(t) + δζ 0 esl(t) + Δζ I (ϕ 1,t) + Δζ Τ (ϕ 1,t) ζ (2) (ϕ 1,t) = Δζ 0 esl(t) + δζ 0 esl(t) + Δζ I (ϕ 2,t) + Δζ Τ (ϕ 2,t) and the difference is ζ (1) (ϕ 1,t) - ζ (2) (ϕ 1,t) = {Δζ I (ϕ 1,t) -Δζ I (ϕ 2,t)} + {Δζ Τ (ϕ 1,t) Δζ Τ (ϕ 2,t)} {Δζ Τ (ϕ 1,t) Δζ Τ (ϕ 2,t)} if the two sites are far from the ice margins and sufficiently close to each other for the glacio-isostatic parts to be equal. The Δζ 0 esl represents the contribution to the icevolume function based on the adopted ice model and the δζ 0 esl(t) is a corrective term to allow for any limitations in this model. This differential observation is primarily a
4 function of the mantle response and, to second order, of the Δζ esl (t). Hence it provides an estimate of the rheological parameters which are then used to improve the Δζ I (ϕ 1,t) and to lead to an improved estimate from observation I δζ 0 esl(t) = ζ (ϕ 1,t) {Δζ 0 esl(t) + + Δζ I (ϕ 1,t)} Usually an iterative process will be used to arrive at the solution. Thus, with a number of provisos, we can estimate the change in ocean volume through time. Regional studies. To successfully arrive at an improved estimate for the ocean-volume function, I believe that it is best to: (i) Carry out regional analyses rather than attempt a single global analysis. The reasons for this include an ability to take into consideration, to a firstorder, the possibility that there is lateral variation in the mantle response function; the possibility of focussing on single data types rather than on merging different data which may bear different relations to sea level; and a ready ability to compare the regional results for consistency or for establishing whether there are indeed regional variations in the sea-level change that are of a non-glacial nature. (ii) Focus on the past 6000-7000 years during which sea levels have been close to present-day values and which provides the relevant baseline for understanding the contributions to present-day and future change. For this Workshop we have focussed particularly on three regions that are representative. These are: (i) The Mediterranean. This is an area beyond the immediate influence of the ice sheets and where the glacio-isostatic signal for the last 7000 years is one of rising sea level and mostly dominates the hydro-isostatic contribution. It is an area of low tidal amplitudes such that the record of past sea levels has a higher probability of being preserved than elsewhere and an accurate reduction of sea-level indicators to a consistent datum should be possible. There is a good observational database, or potential data base, to address some of the specific issues. These include archaeological data and biological indicators of sea-level change. However tectonics are important in many localities and the challenge is how to allow for this. Also, being a nearly-closed basin, fluctuations in sea level from salinity or thermal fluctuations may be important on the modern time scale. (ii) The Baltic. This is a tectonically stable region with the best instrumented record extending back into the nineteenth century. It is generally a low energy environment resulting in a good preservation of record. The isostatic signals are one of falling levels such that past record is exposed and accessible for analysis. The observational data-base here consists of isolation basins. For the southern region there is a zone where the isostatic contribution changes from uplift to the north to subsidence to the south and the data here is less sensitive to assumptions about ice models than elsewhere in the region. A problem may be that meteorological tides can be important.
5 (iii) Stable tropical islands and continental margins. The expected signal here is one of a slowly falling or nearly constant sea level for the past 7000 years and the observational record consists of the age-elevation relation of fossil corals that, when alive, grew in a well-defined part of the tidal range. High-resolution records can be achieved from long-living coral colonies such that it becomes possible to examine the amplitudes and time scales of local fluctuations in water level. Are there others that should have been included? Possible responses or discussion points to the above four questions. 1. What changes in ocean volume have occurred over the past few thousand years since the termination of deglaciation of the major ice sheets? Common wisdom has been that melting of the major ice sheets ceased and that ocean volumes stabilized at around 7000 years ago (~6000 14 C years). In this period and outside areas of glacial rebound and of obvious tectonic activity, sea levels have varied slowly by a few meters. This is largely the result of the global response to the last deglaciation. But the sea level signal for this period is the result of both this response and of any additional increase in ocean volume (whether density changes or addition/subtraction of new water). Analyses of the differential sea-levels as sketched out above indicates that the global rise may have been of the order of 2-3 m with most occurring before about 2000-3000 years BP. There may be another indicator that indicates that the melting did not cease abruptly at 6000 or 7000 years ago. This resides in the change in the timing of the highstands in deeply indented gulfs and fjords/lochs of sites at the margins of the former ice sheets for if cessation is abrupt then the mid-holocene highstands are synchronous. 2. If changes in ocean volume have occurred what has been its source (or sources)? If global changes in ocean volume can be identified from the sea level analysis can the source or sink be identified? Can experiments be designed to aid in the search for the source/sink? Mountain glaciers have been decreasing for the past 100-200 years and if estimates of this are extrapolated over 6000 years then the rise in global sea level could be as much as 1-2 m. Do sea-level records near mountain glaciers exhibit departures from the change that may differ from that predicted in the absence of such glaciers? Mountain glaciers near the current coastline are the results mostly of ongoing tectonics and can this be separated? Why would Antarctic volumes remain constant once the northern hemisphere ice volumes stabilised? Can we expect further evolution of the Antarctic ice sheets in response to the rising sea level? What observational evidence do we have that indicates that there has been a change in Antarctic ice volume during the past 6000 or so years? Uplift of the Antarctic margin? Cosmogenic age data for abandoned moraines or erratics?
6 3. Has the eustatic sea level change been uniform or variable during this interval? An area of recurring debate is whether sea level has oscillated within this period with reported amplitudes sometimes exceeding 1 m. The argument for such temporal variability has gone back and forth since the 1960 s but the early evidence has never been conclusive, being the result of mixing different types of sea-level indicators often from high-tide environments, failure to recognise the spatial variability of sealevel in a constant ocean volume environment, and inadequate chronology. Recently the argument has been revived but it remains unclear that these are anything more than local or regional signals. Do high resolution records exist that enable this issue to be resolved? Are results from different areas consistent? In discussing this issue some thought needs to be given to processes. If oscillations are reflective of changes in ocean volume rather than local changes then this requires not only increases in ocean volume but also decreases. What process can do this on time scales of < 1000 years? For reference, at the time of the LGM the British ice sheet contained enough ice to raise global sea levels by at most 1 m! 4. What are potential sources for new information on high-resolution and high-accuracy palaeo sea level change. The lack of resolution to the above issues means that a greater effort is required in order to improve the observational data base. What would be an appropriate strategy that will lead to the resolution of 5. What other high-frequency contributions to relative sea level occur that may mask the eustatic signal? Can we extract information from the palaeo record on the variability of sea level change on the centennial time scale? Do the coral or vermetid records from single colonies in low-tide environments provide this information? What is the potential for establishing longer records by correlating between individual records from the same region? Recommendations. 1. Identify data types and potential records for the extraction of precise and accurate local relative sea-level signals for the past 6000-7000 years so as to address the above issues, including: (i) Variability of sea level; on decennial and millennial time scales. (ii) Longer term changes. 2. Correlate records from different environments and different geographic areas to establish whether any identified change is local, regional or global. 3. Comparison of the principal methodologies and computational results for the modelling of isostatic contributions to sea-level change under identical conditions.